My friend's stance on it has always been "Fuck it, he ain't your home boy."
Personally, I'd break it off with her and leave it up to her to tell her boyfriend. If she says they have a total truth policy, and you're pretty sure she's lied to him about you....what does that tell you about her? More, what does that tell you about the situation you're about to insert yourself into further?
I try to make it a rule not to get involved with people who are cheating. But I hold relationships pretty sacred, you're either in one, or you're not. I don't believe in a middle ground. So I take a pretty hard stance on cheating.
Bottomline though, if it's about to end, the only reason you want to get involved and tell her boyfriend is to assuage your own guilt. You were cheating with someone's girl, you made that choice, deal with it. Do you really need a punch to the face to let you know you've done something wrong? You don't sound like it. I know what I'd do. I'd thank you for coming forward and being honest, then I'd jack you in the face because you deserve it. Then I'd thank you again for coming forward.
Story time! I met a girl through the Internet, started courting her the day she broke up with her boyfriend. Months later, I went to see her, and while I was out at the gas station, the dude shows up at our hotel room. (This is after calling her while we're cuddling and shit.)
He's 6'4", shaved head, tattoos IIRC, and he takes regular kung-fu classes.
Turns out, after talking with him and her in the same room, he's super fucking cool, a gamer, a nerd and pretty Zen.
The only thing that separated me from an ass beating was that one day of legal versus cheating. Think about it, next time.