We have a Dyson Animal vacuum. It has a bare floor setting and it works great on our hardwood floors. It's so powerful you can see the dog hair being sucked into it before the vacuum even goes over it.

Maybe you and your MIL can work together on house training the dog. First of all, getting him fixed might also help. But that's a whole different battle.
Does she not realize that he is not house trained? And is she okay with his marking? If she's oblivious then it will be very difficult for you to get her to work with you. Sorry I don't have more advice on that subject.

Good luck!!