Seriously, you don't need to borrow trouble for yourself. Especially in this circumstance when it seems like it's not your place to involve yourself by telling her bf.
As far as the second part goes. Everyone does stupid things. I have some friends that have cheated on their gf's. I don't like it, but I'm not going to throw away history and a great friendship because they did something stupid. All you can do is be a friend and advise them to not do things like that.
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"The object of war is not to die for your country but to make the other bastard die for his."
-General George Patton (1885-1945)
Member of CLAN HADB