I do think it’s odd to ask someone who doesn’t have kids for advice about childcare but I would assume she has contacted him because it’s a topic they’ve discussed before and he can give her advice, obviously he’s aware of her family/support system situation.
I have coworkers who have become good friends so I don’t think it’s weird that they’re in contact outside of work and talking about personal topics. If you feel her behaviour is crossing a line then all you can do is tell him. If he doesn’t set boundaries/believes their relationship is appropriate and doesn’t need to change, then either you accept that and move on or decide this isn’t something you’re willing to accept and act accordingly.