I can't keep this secret!?

[ABC]

There is a girl in my group of friends who i don't like for multiple reasons. She wears push up bras (nothing wrong with that) but she then takes pictures of her chest and just fits her head in the picture and people are like 'omg your boobs are huge' and she says i know and when people anonymously ask her on websites she denies wearing them. She put a picture up on facebook, inside of her in her bikini, she might have well just captioned it 'fan pic' and then after a sh*t load of abuse from her 'friends' she deleted it.
She ALWAYS has a guy chasing after her and always has a boyfriend. She has been going out with this guy for 4 months and he's a really nice guy. There is another guy in out year who is mega popular and i'm surprised he talks to her but basically he sits in front of us in history and they text apparently as she said something on facebook to him about texting her back. Now he is the kind of guy to use girls for sex, that's all he looks for. And this girl went on holiday two weeks ago and apparently just before she went away she sent this guy a fan pic, i dunno if she's naked or what but i don't even think she's sent her boyfriend one, they haven't done anything sexually.

I don't know about you but i see this as sort of cheating? It's slutty and vile. Lots of people send fan pics and i don't care, it's the face she has a boyfriend!! One of my best friends told me and she said DO NOT TELL ANYONE because the girl that told her told her not to tell anyone. Apparently all the popular girls know and stuff but no one has said anything out loud about it? The girl gets back from her holidays in a couple days but i don't know what to do! I can't keep this secret i feel sick knowing that i know and none of my other friends who are best friends with her know! I also feel what she's done is sh*t on her boyfriend and i think he has the right to know! I don't want her getting away with it but don't want to pull her up cause she will lie as she always does and she will ask me who i heard it from! Ah help!?