Are neighbors of the tenant, living in a separate house or building? If so, what is in the lease is meaningless to you. It would be up to tell landlord to enforce and he has already told you he has no problem with what is going on. It is not illegal to store things in a garage and it is not illegal to sit in a garage. The police have no authority here and an attorney has no power not a reason to say a word.

The he only possible action you might have is if the guy is violating any noise ordinance that may be in place or some other ordinance that may be being violated.