When you install avast from an account with admin privileges, it should be for all user accounts. Limited user accounts can’t make changes to avast.

I wouldn’t share a PC with this friend unless he conformed with my rules for users.

I guess your friend bought this PC - if so there is even more of a case to say my system my rules if you don’t like it get your own PC.

I would also recommend that your friend ensure that the security challenged friend is only given access through a limited user account.