He may not realize that but if he was looking at his indeed account he’d see that she is actively applying, it would probably suggest her to him for job matches, and he could also see that she has updated and is sending out her resume. All that is a long way to say that it is not a huge breach of privacy or confidentiality to tell him. I personally would. She is about to burn him anyway and look out for herself regardless so you can help a friend and she will still get the same outcome she wants in the end if she wants to leave. The only thing you may be doing to hurt her is cause a little inconvenience before she made her exit anyways but she’ll still go find another job regardless as that is clearly her plan. I’d say tell him.