Is it reasonable to report another student for cheating when it has no impact on me? I currently am studying on an Undergraduate Maths course, in my third year. At my university there are various modules from which students can pick to make up the required number of credits for the year. 

5/6 of my circle of friends take a particular (popular) module which I do not. Recently, they had a piece of coursework due for submission. They had about two weeks to do it, and the submission deadline has now passed. 

One of the students, with whom I am friends, has informed me that she took pictures of her answers and gave them to another student (with whom I am not friends) three hours before the deadline, because that student had done little/no work and was going to fail. This was not a group piece of coursework, and she essentially gave the other student her answers.

I'm not entirely sure why I'm so upset about this, considering I don't take that module and it has nothing to do with me. I have my personal opinions about the student, but I feel like that shouldn't be relevant to the situation.

I do want to report this as cheating, but I'm also well aware that it's effectively hearsay. None of the others of my friends (who do take the module and who are also aware of this incident happening) has reported it. 

I'm wondering whether it may even be out of line for me to report this incident, given that I have absolutely no evidence in my possession of what happened, and I do not take the module, and I wasn't involved in any way. 

Is it appropriate in these circumstances to report what I've heard to the lecturer of the module?
As others have already mentioned, cheating is wrong and it's the right thing to speak up when someone commits a wrong. It may also indirectly affect you when someone cheats (eg. through grade inflation).

However, by reporting the cheater you are also implicating your friend who passed on the photos and facilitated the cheating. Your friend might feel upset (and perhaps rightly so) that you didn't put your friendship above some general (and perhaps minor) moral obligation. I caution that your obligation to report is not unconditional in the sense that you have to accept a disadvantage for yourself, such as a spoiled friendship. Moreover, I would argue that you also have an obligation of loyalty towards your friend; whether or not that 'trumps' your obligation to report depends on how close your relationship is.