Legal Question in Criminal Law in California

Yes, she can (as well as for battery), however, I think it will have very little merit. What are her damages?

She can sue, but the important questions are whether she can win and, if so, how much.

It's legal to use force in self-defense or in defense of others. But what is reasonable will depend on the circumstances. Merely restraining the mother was probably fine. But if he injured her in ways that were foreseeable, then he may have gone too far -- unless the mother seemed intent on seriously injuring her son and the friend could not reasonably have tried to use less force.

As Mr. Hill notes, the extent of the mother's injuries will play a large part in her decision about whether to sue. Lawsuits are expensive, and lawyers will only work on a contingent-fee basis if they think the case will result in a large, collectible judgment. If the mother was seriously injured and the friend has assets and/or insurance, then she might have a decent case.