Is it permissible for a pregnant woman, who needs to eat and is unable to do so in her workplace because of her manager, to eat in the restroom out of extreme necessity, knowing that her health is affected by not eating?

What the manager is doing is contrary to the manners a Muslim should observe with non-mahram (definition) women. Speaking with them should only be out of necessity and to the extent of that necessity, without undue familiarity. It is also incumbent upon the woman to prevent him from exceeding bounds if anything of that nature emanates from him.