Fleas are VERY difficult to get hold of since they hop quickly out of sight. They are very tiny black things that would fit on top of a straight pin. (the top part that's flat).
Does your room mate allow her cats outside? Cats that are allowed outside all the time will surly bring fleas back with them.
Buy some diatomaceous earth and follow the directions putting it down inside. It will not harm pets at all, but it dries up insects.
Food grade diatomaceous earth for flea and tick treatment.
Next, just mix up some all natural ingredients to mist on the pets. Here are some fantastic recipes that really work:
Chinaroad Lowchens of Australia - Ticks, fleas, bees, flies, snakes and other gremlins! And how to deal with them!