Something in fact had happened (she's allowed someone who is not her boyfriend to penetrate her) Even if nothing else had happened but that, it is still something to be concerned about. Would it make a difference if she had had a full blow intercourse with him? The sexual cheating is still that- her breaking your trust.
The question is- how willing you are to forgive and move on (you said everyone makes mistakes, and if you are willing to live with that and give her another chance, then do it. If you believe that what she did is absolutely not acceptable mistake or no mistake, then figure out what would be better for you. Don't you think you've already lost her based on what she did? You won't be able to keep her if she does not want to be a loyal partner finger penetration or anything else. Even kissing someone who is not you is not appropriate.