What do you mean by 'on the toilet', like sitting on the toilet (toilet as in that porcelain thing?) That would be på toalettstolen or in ordinary speech, på toastolen.
badrum is only 'toilet' as in 'the room where the toilet (the "porcelain throne") is' but toalett is both kinds of toilet. And toastol or toalettstol is only the porcelain thingy.
(of course badrum can also refer to a 'bathroom' with a bath and/or shower but no toilet).
Can this sentence refer to an event that occurs usually, like saying that 'she sings in the bathroom' most of the time? Or does it refer to a one time event?