Why hasn't he said anything to her? There is the rub. Its important to say what's going on and how one feels as soon as possible. It lessons expectations and assumptions immediately. No guessing, no intuative *knowing* it is all out in the open and dealt with. There will be stuff that is hard to go through but its harder to go through if its left because of fear. Fear creates more pain. I would feel disrespected if I was not privy to how people feel and what is going on for them. Especially someone so close. I would feel less connected and honoured.
At least this has been my experience.
If I were him I would spend some time telling her how much she means to me, how its not going to be anything more than a friendship and that it doesn't mean that they can't be close friends. In essence he has a poly relationship of the non- sexual kind with her!
same rules apply to friendships as they do to intimate relationships as far as I'm concerned.