Legal Question in Family Law in California

No, it isn't illegal. In fact, she cannot change her name back to her maiden name without a court order.

Yes. But problems can arise if the woman is doing more than just using the ex husband's last name.

I have an ex who continues to use my name almost twenty years after we got divorced. When I remarried, I changed MY name because my last name used to be Beer and I married a woman named Ginger (I swear; I can't make this stuff up). Now I have an ex using McCormick. Sorry, there's nothing you can do about it.