a company can cut you off instantly for any reason. I say this as a person who's let multiple people go — not for cause, but just for RIF purposes. She should be doing whatever she can (within the realm of honesty) to convince them to hire her, and she shouldn't be giving them any reasons to think she won't be the best hire they've ever made. She should only be telling them good things that paint her in a positive light. No way should she tell them she's pregnant.
Relationships with companies are meaningless. And I happen to love where I work. But they earn no special loyalty from me. My obligation ends to them the moment I get my paycheck every other Friday, and their obligation to me ends at the same time.
Tim