If I were you, I think I would do more disciplining of the boy. I would kind of act like I would if the mother wasn't there. For example, if he came to push my dd off of her trike, I'd say, "Dd is still using it, you'll have to wait your turn. Here, want to play with XYZ?" Have you tried that? I think your dd would appreciate you standing up for her, and you can model it for her so she can learn it too. In fact, you could even tell her, "Dd, if you're not finished with the trike, you can tell him that you're still having a turn. Say, 'I'm not finished, don't push me!'"