it's inappropriate as hell.
the dudes are just waiting for an opportunity to fuck the girl.. bottom line. it's true for dudes in relationships, and ESPECIALLY true for her friends who are single.
if the opportunity presents itself, and he's drunk or she's drunk, or they're both drunk or whatever, something will happen.. maybe not sex, maybe just a kiss, maybe a hand down her pants... whatever... it'll happen sometime...
... and when you find out about it, you'll never trust her again and the relationship is forever tainted.
if she knows you're not cool with it, especially when you explain to her WHY... and she refuses to do anything about it, you gotta leave her because she knows what's gonna happen as well and she doesn't wanna stop it. she's putting her 'relationship' with her friends (dudes that will talk down to her about you in a heartbeat) ahead of her actually relationship with you.
a girl like that doesn't deserve your time. find a girl that's respectable and who respects you.