A.C.
First, if he's not comfortable talking to her, respect that. But do insist that he be polite--instead of running away & hiding, just tell him to go play in another room.
The best thing you can do is acknowledge and respect his feelings--even if he doesn't know why, it's okay for him not to want to talk to her. And if she says something about it, explain to her that sometimes, it takes a while for kids to feel comfortable with new people.