Quoting Stephanie & Ryk:" I really hope not. I know hes hit his daughter over petty smurf. "
Not cool. I would ask him casually, hey the baby has a red spot on his back, do you know what happened?
With abusers, it normally only escalates further and further as the child gets older, especially once they learn how to argue and talk back. I honestly would not let the child return unsupervised.
Don't trust that your child will tell you truthfully what happens when gone. Children will protect a parent, especially if being threatened, scared, or coerced. Just be careful and keep a very close eye on the situation.