See I find it kind of weird that you are so cool with the guy who your girlfriend has allegedly been cheating on you with o.0
Haha...same. I don't know about other people, but cheating to me means involuntary hatred towards the person your mate is cheating you on. Also, Jerry Springer images pop into mind.
That's not to be taken lightly, if she "did" ( as you've convinced yourself ) cheat on you once, what's stopping her from doing it again? You should wait a few days after you call her ( assuming you already have ) and wait for her to call YOU. Make her suffer, inhumane but effective...let her see how important you are to her, maybe realizing she commited a grave mistake and that she needs you back.
And by you getting along with the guy shows you don't are too forgiving, a huge defect that will probably show if your relationship continues. It will probably just hurt you more in the long run.
By the time you've read this the drama must already have been dealt with >.>