I agree that it’s the son’s choice and that she oughtn’t to have invited herself.
But just allowing myself some mom-type thoughts here for a minute, I’m focusing on those words “for safety purposes.” Her words, right?
Those words suggest that there is something to fear, at least in her mind.
I’d have to ask if there’s any reason to think that he’s (a) dangerously overdoing his drinking, (b) using drugs, and/or (c) in the habit of doing reckless things while intoxicated—things from which she might have rescued him before. Maybe even things from which he expected her to rescue him before (overdose? arrest? inability to drive safely?).
She might be worried sick about what he’ll do if he goes to this blowout event on his own—and she might be perfectly willing to be called all the things people will call her and sneered at and shunned and all the rest, in order to be there and do whatever she can to protect her boy.
I’m not advocating this approach at all, you understand. I’m just saying there could be a mother’s side to the story as well, and she might have reason for her attitude.
If this is the case, she needs help as much as he does. The help he needs isn’t hers, and the help she needs is in taking care of herself and not trying to manage someone else’s life. Hard lesson when you’re scared to death.