Personally, 19 months is a bit young for me to trust them to be careful with it. Especially considering it's a collarbone - as others have said, it doesn't get a cast, and they can be difficult to heal unless the child keeps it still. It's easy for a child that age to get excited about playing and forget that they are hurt.
I might suggest mom see about having a grandparent or friend (with fewer kids) watch him for a few weeks. If there's no chance for back up, I'd take him, but strongly suggest he take a bit of a break.