I say just tell him exactly what you just told us here. You don't want his dog to get hurt. If he can't keep her under control you'll have to tell authorities to help protect your own dog. I don't know anything about dog laws (even in my own state/town), but couldn't you report this dog and if anything did happen to her by Mikey they could see you were being proactive in trying to remedy the problem?

But you should really tell him and express how Mikey reacts to her and the (obvious) fact that Mikey is much bigger than her and could seriously injure her and you do NOT want to see that happen.

Good luck!