1. Is it legally permitted for a boss to say this?
Absolutely not. He also can’t legally make you cover your hair when married, unless it is a dress code rule (which unless men do it as well, it would be sexual discrimination).
2. Uncovering the hair for a divorced woman (with children) is it so simple to say that she can uncover
No, but Hetairim (and good ones) exist, as others have pointed out. Ask your LOR.