She should keep on eye on her son and his new friends but should never tell him he’s not allowed to hang out with them cuz that will just make him resent her. The best way to get to know his friends is by inviting them to your house and you’ll probably see they are good kids who are confused.
Also, like others said before why do you think he is hanging out with these kids and if its recent what caused it?
Another thing, don’t be too overbearing and always asking him where he’s been and what he’s doing. Give him breathing room but support him. Be there for him cuz he’s probably going through a rough time.