I think the best thing to do is just end it man. Don't mention Aaron, don't try to make any witty remarks, it'll just make you look petty, and make you look like you got hurt (even though you did get hurt, but it's probably better not to let her know).
There's no point in trying to make her feel bad. Look at it this way - clearly she doesn't feel bad about what she's doing, otherwise she wouldn't be doing it. She doesn't value you or your feelings half as much as she says she does - if she doesn't feel remorseful about cheating in the first place, why would she feel remorseful when she gets caught? Confronting her will only annoy her and she'll try to put the blame on you somehow and make you out to be the bad guy.
Just tell her you don't want to be with her any more, say it's not working out, cut contact and walk away before any drama starts. You'll feel better for it in the long run, and you'll avoid saying or doing anything stupid in the heat of the moment.
Also, try to forgive her - and when I say this obviously I don't mean accept her behaviour and be a doormat - I mean try not to be bitter about it. Don't let sour feelings hang over your head. Just realise that she did something bad to you, you don't want that in your life, get over it and move on. She can be Aaron's problem from now on, take pity on him because she'll do it to him too.