Tbf, being a foxgirl she probably knows how to use a toilet. Just…yknow…don’t go in there for at least a few hours after she’s been in there.
With that line of thinking you could also argue that, since she has the body of a human, it can be assumed she doesn't have the excretory system of a fox so the smell wouldn't even be produced.
But that just brings up the big question: How much of a fox girl is fox?