should I forgive her?

Your first reaction was just normal my friend... But your lucky enough to know that your girlfriend admitted that she was going out with another man. It would be better if you will confront her to talk and clarify things out. Not every man gets an honest girl like you have... Why don't you ask her if she really love that man over you... Then if she replies, you can perhaps talk and negotiate, in order to avoid further damages done... Hope both of you can be in good status already... I hope for it...

YOu wanted to help her from her other guy? Hmm... If you made a big mistake that made her have another man I guess that you should not hate her but if she really wished to have another man or if she loved the other man than you as her boyfriend. I guess you just need to forget her. Your statement wasn't that clear to me. I wonder what was her reason why she did cheat on you?