Well I think since Janet is to some extent capable fo projecting a magnetic field of her own, he'd have a bit tougher time than with, say, Wolverine, but he would still be able to slow her down significantly, possibly to human levels I think. She doesn't use any electronic that would be affected so at least he won't be wiping out her hard drive and cheaty stuff like that :P

Also KL, you evil man! You robbed us of our fight :/