I agree with ADHDmomma that natural consequences are best. Whatever he does, he needs to feel the sting of the repercussions of his choices (and not be shielded from them) and he needs to be responsible to make any wrongs right, whether it’s through money or some other way, which is sounds like he’s been doing.
Unfortunately for him, if he continues this behavior, one of the natural consequences might be just what you said you fear: losing friends and encountering parents who may not be as understanding. There isn’t much you can do to keep that from happening–you can’t change his behavior for him. But, maybe those kinds of hard consequences are just what he needs in order for him to really see the importance of learning new skills and making different choices (and being willing to get help, like taking medication, going to counseling, working on etc.) Better he learn now at his age that “if I do x,y,z I will lose friends and make adults really angry” than to wait til he’s an adult and suffer worse consequences than this, like making a boss mad or alienating a spouse beyond repair.
In terms of how long you should ground him for this latest issue, you might want to approach it in a different way by calmly saying, “It seems like you are not able to control yourself over there. Until I see evidence at home that you’re able to manage yourself better, you will not be able to go over to friends houses.” (i.e., you explain that you are removing him from a situation that he is not prepared to handle– not so much as ‘punishment’ but really just because he obviously can’t control himself in that environment so as a protection to him and to the other family, you have decided it’s best to wait til he’s “ready” to be over there again. You can tell him that when he proves he can manage himself better, i.e., when you see at home that he is much better able to manage his moods, impulses, etc. then you will allow him to go back over there.) In the meantime, you could allow him to spend time with friends at your house when you are home to keep an eye on things and intervene if things get out of control.
Hope this helps!
Parenting Coach, school counselor, author, mom of adult son with ADHD