She seems to be in denial, so she will not accept anything that you say. She probably has no evidence to say your boyfriend has cheated, so just ignore her because she is just trying to make you feel hurt (probably because deep down she knows what you say is true and is hurt, but is denying it) and eventually it will all end in tears. The more you warn her, the nastier she will get, so be careful if you mention the subject.
The way I get out of situations like this is to let the person sort it out for themselves. It will end in tears and there will be heartbreak, so make sure you are there for her when this happens. I would not over-voice your concerns until this happens, but when it does you will be able to speak to her about it and explain to her how it hurt you to watch her doing this, and about her nasty comments.
She is risking her health and could easily get pregnant, but until something happens she will continue to deny it and unfortunately she will not listen to you.