Should I tell my crush he's being cheated on?

Well, here's my story. Me and this guy used to like each other and he never told me but he initiated it with his behavior and everyone would ask us if we were a couple and stuff. he kinda led me on though cause when I told him for sure I liked him he told me he was confused. then two-three days later I found out he'd started going out with this girl he'd made out with some weeks ago. I was very upset but now we're supposedly ok cause he said I'm his best friend and he doesn't want to lose me. so I'm trying to be ok with him.
but and. this girl. she's cheating on him. and I know. cause the guy shes cheating on him with is my friend and he told me everything and told me not to tell her boyfriend.
but now I don't know what to do. if I tell him she's cheating on him he's going to think im the 'rat' trying to separate them, which is most certainly NOT the case. and also my other friend's going to get pissed off at me for telling.
but if I don't tell him then hes going to keep living in ignorance thinking that his girlfriend is oh so innocent and deep down inside I WANT to prove him wrong. And if he ever finds out hes going to hate me for not telling him..plus I dont want him to get hurt..:/
so what do I do?

I say leave it alone
he will figure it out in the long run
u have 2 friends on the line
so you shouldt do nething
if you were to want to do something
go to his girlfriend and just say ' I know'
eventually her guilty concoius will get to her

Don't get involved in rumors. Unless you K and O W that someething has happned. Tell the person spreading the rumor to go back to the person they are talking about and get permission to talk about there personal business. 99 out of 100 times that will end the rumor and keep your integrity. Treasure your friendships, don't surcumb to the temptation to get revenge for your hurt. Grow thru these things. You will come out of it an even better person and still have the friends. Good Luck Gino

Unless you know for certain that she is cheating, if you catch her or the guy tells you direct then leave it alone. Becos you are right if you go to him its going to look like your trying to break them up and you'll look like the bad guy. Just leave it alone, if she really is cheating she'll start to show signs eventually.

just don't do anything and there will be no problems