Not only is it unacceptable what your so-called boyfriend has allowed to happen, but it is doubly unacceptable that your friend had taken it upon herself (drunk or not) to make a move on your boyfriend- grinding and kissing him.
You should not feel guilty at all, and it is digusting that your friendship circle are also siding with her and not you. Presumably, they know that what she did was a B***h move. They seem like the sort that love drama and love having fun at others expense, these sorts of people are not mates.
I'd find out from your boyfriend what happened, and then ask your mate and wider circle. Once you've found out I'd probably just cut your mates and friend loose. You've only been with boyfriend 2 months so perhaps he didn't know what he was getting himself in for? Is he a 'jack-the-lad' sort or reserved?
Its hard for me to offer any valuable insight on your boyfriend. But certainly your mates and friend are wholly ass****s. Nobody does that to a mate, regardless of how long you've been with him.
Your friend obviously loves herself, and I've known/know people like her IRL and to be honest, not worth the time of day.
Cut your losses- get rid of them, depending on your boyfriends responses- tell him/them that their welcome to one and other and find yourself real mates.
At this young age of your life, you should be enjoying your life, and real mates would be fulfilling your life, not bringing you down. Your mates are simply siding with someone based on their own insecurities. They are probably all envious of her and want her looks or something and perceive her value as a threat. So naturally they side with her over yourself. Bully mentality really.
Chin up
kick them all to the curb. I'm sure your equally as wanted as her, and you could find yourself somebody else with at least a shred more decency. Same goes for mates. All the best