Alright this will be a 1st and long post but I'll try to condense as best as I can.

\-----------------------------(Story/info) 

So I (25M) have been friends with my best friend (26M) for about 5 years. We work together have similar interests and world views. We had also been living together for the past 2 years. Up until recently I actually looked up to him and thought he was honestly the most well rounded person I had met around my age.

A little over a year ago I started dating my SO (21F) a few months in for convenience she essentially moved in. Things had been going pretty well but we started having some issues but we were keeping it together. As background for the upcoming story she is also easily the biggest lightweight I have ever known. anything past 4 regular light beers and she starts getting trashed. 

So about a month ago we're all drinking sitting in the living room and I've gotten to the point where I want to taper off and start going to bed. It's around 2 or 3am and my SO has work the next day while I do not. I recommended she come to bed so she doesn't feel too bad in the morning. She's also getting to that point of no return level of drunk where shes slurring pretty heavily, laying in awkward positions, and general tells of someone being very intoxicated.

She tells me she thinks she should just stay up at this point so she doesn't accidentally sleep through her alarm. I guess that makes sense I tried to convince her otherwise but generally shrug it off and go to bed.

I woke up the next day and everything seemed alright but my SO eventually came up to me later in the day and explained that her and my best friend and fooled around the night before. I was upset but I've been cheated on before so I wasn't hopelessly crushed. I went to my best friend in the other room and asked him what happened he tried to say they only made out. She told me different but I didn't let him know that. I also asked him if he planned on telling me and he shrugged his shoulders and said hey man I dunno its awkward.

Over the next few days the SO filled me in on some more details that I asked for to get a better grip on the situation.

Essentially most of the time was harmless till he came over started putting the classic moves that you do then proceeded to talk badly about me and how I am including some stuff that feels completely made up while he talked her up about how nice she's been ya know... the classics. She was the one to eventually stop what they were doing and walk away.

I condensed alot of it but if the specifics of what happened are needed I can edit it in later or make another post later.

\---------------------(What I have done)

So knowing all that I can tell you how I've been handling it.

For my best friend I've done everything I can to avoid seeing him or acknowledging him in the apartment. I've made plans to move out as soon as I have the place available I'm gone. I felt I gave him his chance to explain his side and he shrugged his shoulders. He also didn't say another word or even a sorry till 3 weeks later while he was shitfaced. I can forgive mistakes even monumental ones like that but lying about it then not planning on telling me and the way he went about it just seems sleazy and malicious. also he is quite a heavy drinker so he can handle his drinks and he wasn't that bad off when I went to bed.

For the SO at first I told her to be moved out by new years. She was honest she knew she fucked up and she hated herself for it. She beat her self up enough so I was perfectly content with acting like nothing happened until she moved out to allow her to have a smooth transition. Recently though we have been talking about possibly working things out and I'm just very unsure of it. 

On one hand she was honest with when in reality that's a hard thing for most people to admit to someone knowing full well you'll earn nothing but that person's anger and hate in return. On the other honesty can't get you out of everything nor can alcohol as an excuse and if we continue on it will be very hard for me to remember this and have it lorded over her when she might be acting shitty towards me. I know that isn't right but I'm human too and I don't know how I might feel later down the road. I also feel that maybe the part that wants to work on it is just being weak afraid of being alone after losing the only 2 people I really interact with outside of work. If that's so then that's unfair to her.

I've talked to some friends at work about it which is the closest petty thing I've done to the best friend however I legitimately wanted their opinion (2 older males) but part of me did feel like I was exposing a fraud. They were surprised considering how close we we're but they think I should give him another chance or some sort of leniency.

\-------------------------(TL;DR)

So after all this AITA for completely shunning my best friend while forgiving my SO and possibly letting her back into my life fully