I'm in university and happen to be a chemistry major. I don't have a lot of friends because I'm working a lot, but one friend in particular (we'll just call her friend for simplicity sake) shares lab class with me. We met in our chem lec because she just sat next to me and we started talking one day over something we didn't really understand.  
Now, Friend isn't the smartest. She's a Biochem major. Out of the two of us, I'm the smarter when it comes to Chem but she smokes me in Bio because I can't wrap my head around some of it. We all have our strengths and weaknesses.

Anywho, we were lab partners. We didn't know anyone else in the class except for this guy who I'll call Al, and Al was always late to class so he wasn't there when we picked partners. We just picked each other for simplicity and having the same lecture class. This is when we started being friends.

We worked on the HW for both classes together usually. In the start of the year, it was easy review stuff; nomenclature and dimensional analysis and other more 'basic' chemistry stuff you learn in university. Slowly, things started to get harder and we got into more complicated subjects, both in Lec and Lab. When we would do HW for either together, she would want me to figure out the answers and give them to her. Sometimes, Al would come and do HW with us; he was also Biochem and struggled with Chem and would sometimes mooch answers off of me, but most of the time he just wanted help on how to do a calculation or he just forgot something really simple. Friend didn't seem to want to put in the work, but we were buddies so I helped her. In lab, it was the same stuff. She wanted me to do all the work. She had awfully shaky hands when handling glassware so I would do all the carrying glassware related things so that we wouldn't accidentally break a beaker or a glass pipet or something; I get it. My sister has early onset arthritis so I was used to handling stuff that people would shaky hands were too scared of breaking shit to do. But I was doing everything. Weighing out stuff, mixing things, you know all the chemistry lab shit from high school. I tried to get her to do things but the only thing she would do was light the bunsen burner (I was a hapless idiot who couldn't do that, the easiest thing, for some reason). 

Time passed and I stopped giving her my lecture and lab answers but would rather help her. She would try to get me to just tell her what was going on but I talked to Al about it and he gave me what seemed like solid advise. Just tell her to use the book, use youtube to look up tutorials, Khan Academy, whatever. So I did, because I had other shit to worry about than trying to talk to a brick wall. Like, I had 1500 word essay due every week for one class and tons of other stuff, and I suck at writing papers. I felt bad but it is what it is.

She started going to our lecture prof's office hours for help with that, so she began being okay with that. As the lab practical inched closer, I really tried to teach her what was really going on in lab as a last-ditch attempt to save her grade. She never let me see her lab grades after I stopped giving her the answers but I happened to see one when our lab instructor handed them out. He grades out of 25 pt scale, unless it's a dry or double lab. She was getting 12s and 14s, so I felt guilty. She was my friend, I didn't want her to fail. So, like I mentioned, I really drew out our labs for the whole class so I could teach her step by step how to do stuff and make her do it. The only thing I didn't let her do was carry glassware after she shattered a 600ml in the sink because I didn't want her to break anything else and hurt herself or someone else. Luckily, she was wearing gloves so she was fine, but whatever.

Anyways, lab practical comes around. We do these as single people, no lab partners, at different times. I go first around 9 pm and it goes pretty smoothly. The lab practical is basically 4 short versions of lab we had already done that had crucial techniques we had learned like... forming a precipitate and what causes it. Pretty basic shit. I finish in with a half hour left to spare and I feel confident about everything but the precipitate stuff because I was pretty sure I accidentally contaminated the wells I was using but I didn't want to run out of time and kept going. Now, you're not supposed to tell your lab partner what labs were on the test but, in a last ditch effort, I totally did. Told Al too, but his lab partner already messaged him. Whatever. Word gets around.

Now, Friend gets extra time on tests because she can't do math as fast as most people. Luckily, there wasn't much math on this, but still got a half hour extra (3hr30min in total). 

She ends up not finishing 2 of the labs and not getting to the bonus questions (which were hard if you didn't know what you were doing, I got 1 of the 3 right) and I feel... terrible. She gets upset with me, saying how I always did everything in lab all year and ditched her and never helped her with the lab reports or lec hw so she didn't know what was going on. I end up getting an A- on the lab practical and she doesn't pass. I don't know what her letter grade was but she got a little more than half of the points and it was a 100 pt scale.

I felt really fucking guilty. I didn't have many friends so I ate alone after that for the most part because she wouldn't respond to my texts. She only came to me when she wanted to study for the lec final (Al came too but acted like a dork the entire time-he somehow didn't remember the difference between a solid and a liquid and I worry for him). Friend was all buddy-buddy during that but wouldn't talk to me after the final happened, despite us walking there together. She had to take it in a separate building because of her extra time but those buildings are right next to each other.

I don't know what she ever got on the final. We're still friends but she sometimes brings the lab practical up and makes it seem like it was my fault for her failing, saying I hogged the procedures and wouldn't let who do anything so she never learned. I was fed up with her mooching off my answers and I knew she wouldn't have learned anything if she kept doing that, but her grades only suffered and she took a bullet on the lab practical. I'm pretty sure it really harmed her GPA and I don't even know if she got credit for either course. If I would have given her lab practical answers or lec hw answers, the final and practical wouldn't have hurt her that badly maybe? I don't know, I just feel like a dick for letting her go off and just... you know, fail and all that. I'm pretty sure I'm a shitty friend because she gives the implication, but I tried my best. Al said I was kind of a dick afterwards even though he gave me that advice in the first place. 

Thanks for your feedback.