Is it my place to tell the truth?

Okay, a bit of a long story here. Essentially, I've been really close friends with a guy for about 1.5yrs. 4 months ago, I broke up with my boyfriend of 3 years, and then about a month after that my friend and I pulled on a night out. That provoked us to both admitting that we really liked each other (he especially admitted that he has always sort of liked me like that), but I stressed that I wanted to remain single because I'm enjoying the independence that I haven't had in a long time. For about a month, we had a "thing" since we were seeing each other loads, sleeping together etc. That died down because he got stressed about uni and job interviews, so I gave him space. He took that as me no longer being interested in him, and it all came to a point when we had an argument because I got upset when he was texting me telling about girl who'd asked him out on a date, since he didn't realise that I had still had feelings for him. Since then, we've been acting like good friends (ie. for the past 6 weeks or so) until last weekend, when we both got quite drunk on a night out and went home together. Whilst us sleeping together on the night can blamed on drink, the morning after we slept together twice more, so it's left me feeling a bit confused. Now here's where things get really complicated...there's another girl. He's never told me about her, but since we have mutual friends it's come out in conversation that he's been sort of seeing her since about a month before I broke up with my ex (ie. 5 months ago). I didn't care at the time since my friend and I hadn't even gotten with each other when I was told, but someone told me her name so I do know she pops up on facebook and stuff. Whilst my friend and I had our "thing" it was quite obvious that there wasn't another girl, as he was texting me constantly and always asking me to see him so him doing that with another girl as well couldn't have been possible. Since we've died down, it's become apparent that he's started dating her again. She's clearly smitten with him as she's always popping up on his facebook and commenting, even if she doesn't get a response. Whilst I still have feelings for him, I don't take issue with this because it's not like we're an item or anything. However, today has really shook me up as I've inferred from facebook that she stayed over at his last night, which is only 6 days after he and I slept together. I feel awful for her. Even if nothing else happens between my friend and I, if I were in her shoes I'd still want to know if he'd slept with someone else so recently. What gets me about this is that she clearly really likes him and has no idea about what he's got up to behind her back. I'm not even angry for myself because he more or less jumped at the first opportunity he got to get with me since I'd been single for a short period of time, and then when things died down with me he went back to her. Should I say something? I know I'm sacrificing my friendship if I do that, but the way he's treated her has made me so angry.

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