I will start by saying that I am not an advocate of homeschooling for one reason... it is VERY difficult to do it correctly and people do not understand that. I have seen it done well and have seen it blow up b/c parents do not understand that teaching is not something "anyone" can do- many parents have a hard time stepping back and asking themselves whether or not they have the patience, time and interest in homeschooling.
When she calls, I would ask her what about the situation bothered her- was it the fact that her child was pushed or the way that the school handled the incident. If she's upset with the way the school handled it, call and schedule an appt with the principal, teacher and the other parent to discuss what happened and her concerns.
Pulling him out of school will not teach him how to react to situations that he WILL encounter in his lifetime. It won't teach him to stand-up for himself, nor will it prepare him to be a community member.
Point out that he's little, but he's not too little to learn how to be assertive and use his voice. If she wants to pull him out, I would suggest having her contact a local parent who is homeschooling and spend a few days observing and meeting with her local school district representative before making a very rash decision.