Its not normal behavior per say, it is, but he knows whats there, he wants some! Just jokingly say stuff like 'ooo he likes you a bit much tonight' or 'bet he wishes he could have some of you' something like that when hes checking her out down there, just jokingly put it out there, see for her reaction, break her in to the idea this way and then eventually bring it up and ask if she'd ever be up for trying it, maybe send her some zoo porn as someone else suggested, just nice pictures to see what she thinks, but I'd jokingly mention it first, plant the idea, get her thinking on it