Well I mean, #1 you find out if she actually has a boyfriend or not because the answer to that question should dictate what you are going to do, but there will be men who's answer will be the same either way.
If she does have a boyfriend, then you know for a fact that this woman is a cheat and on that basis alone, I suppose she is capable of just about anything including a rape allegation against whoever to save her relationship.
I mean, if you have questions about if she is a cheat, I could see still fckin with her but if you know for a fact, being the other man, you should leave her.