Henry, dear, Henry.
make very sure it's her fault before you accuse her. my house usually looks like a bomb went off - in every room - but i don't have mice. how old is your house? are you sure they didn't find lots of inviting openings they can use to come in from the cold?
maybe it's the house. but, if it's been checked over by professionals and they've confirmed it's the roomie, toss her out. she's not a friend if she risks your health rather than pick up her mess.