What do i do?! Bitch in friendship group?

[flatbadge722]

Ok, so there's this girl in my group (lets call her Sam) and no one reallllly likes her, like, they don't hate her n they still talk to her, but she wouldn't be their first choice for anything. Me and my other friend (none of these names are real) Brooke, would be the ones who dislike her the most, but we still talk to her cause she's not always mean, she's actually really funny. We've known her since year 2 so we're pretty close, there has also been SO many close calls of ending our friendship. Anyways to the story now..

There's this guy named Alex that used to hang out with us n he was pretty funny though he had to move schools (pretty sad :/) but we still kept in touch on Facebook n stuff. Alex n I stopped talking for like half a year, and just recently started again and we r as close as ever. So he told me something about Sam and told me not to tell ANYONE (haha that went well seeing as I'm putting it up here) anyways, he sent me a conversation between him and Sam, (Sam doesn't really like Alex cause he practically hates her, and this was also their first conversation in probably since he left). He asked how everything was going and she said, 'i sometimes hang out with other people, oh and dont tell Amber (my best friend, she is the nicest person ever and none of what Sam says is true, she is just so nice and caring) but shes such a slut now....' and Alex is like 'what? Why?' etc, and then she said, 'yea..shes just like such a bitch and everyone hates her (everyone loves her) like just imagine the biggest whore you know times it by 3 and add a sprinkle of slut and thats her.' Like wtf?! I knew she was bad, but not THIS bad!!! I'm sooooo pissed off!

Also, Sam was just going on not long ago about how happy she was that Amber and her were starting to get closer n stuff.

I need advice on what to do?! Alex told me NOT to tell ANYONE cause it will spread. Which is true..I wanted to confront Sam myself n tell her to stay away from Amber but Alex told me that she will spread rumors (which, btw everyone would believe my story over hers, cause like i said before, no one really likes her) or get really mad at him.

I really wanna tell Brooke about it, but im scared she'll tell Amber, and i DO NOT want her to find out cause she is really fragile n it'll probably just break all of the confidence that she's finally gained.

Sorry this is so long but please help!
Also, Amber likes Sam cause she thinks they're getting closer :/ and if I told them to stop hangin out with her and ignore her, they would ask for answers, ESPECIALLY Sam. My groups curious c; Oh and also, none of us hang out together, we're all in different groups, but we r all really really close.