Do they have a key? Even if they do, your kid's friend is a legal adult. Anytime someone who isn't his roommate pops in, have him call security or the cops....that'll change their behavior.
Not sure of the exact scenario. I’m assuming their kid is letting them into the building at least as most dorms these days require an ID badge to get in.
I’m sure there are things that can be done eventually, just sucks for an 18 year old out on their own for the first time to have to deal with this. Though also a great early learning situation.
The one other complicating factor is that the dad is a very high powered lawyer. So there’s a little trepidation in how to handle an unhealthy helicopter parent who may have tremendous clout/resources.