How can I tell my girlfriend I don't like her friends without upsetting her?

People who socialize with substances usually seek out others who do, too whether they are 20, 30, or 70. What you really want to tell your gf is that you don't like her (not the friends) when she's trashed, but it's a lot easier to blame them for her lack of control. If she controls it at home, she can do it elsewhere. People stop that kind of behavior when there is a consequence that is so bad that they re-think their posistion. If they all get arrested it will probably be a pivotal moment. It's her choice - the friends aren't holding her down pouring booze into her. I hope that you have big enough nuggets to tell her the truth, before one of those pesky consequences shows up.