what is a good mousing cat?

Any cat can catch mice, theres no particular breed. Usually if the mice smell a cat around they normally just move house if the cat dosnt catch them first.

a good stray, get an adult, since kittens can't hunt until they are bigger anyway. In this case, any short haired cat is libel to be better than the longhaired/persian types, tho I can tell you that the longhaired/mainecoon types are great mousers. Siamese types, long legged, slim, active types are also good mousers, any alert, active, curious cat will be a good mouser.

I have to say that any cat will kill mice mostly because they're great for them to play with. If she really wants to get rid of mice then I'm afraid poison is the only way. Personally I'd of course go for humane mouse traps.

A no-breeding cat. An alley cat would be better to chase mice away, specially those cats with pointy noses. My grandmother said that cats with pointy noses are very good hunters. Try a dark colored one, my godmother's cats' vet said dark cats have better health and live longer, I don't know why... but she said that.