My housemate just bought a pet snake...?

Oh don't worry about it, pet snakes are as harmless as pet dogs or cats, maybe even more harmless than them if you don't bother them. If you stay away from the snake and that tank you will hardly remember that it's still there. The snake would most probably not end up in your bedroom if your housemate has a secure enough tank. As for the live mice problem, just ask your housemate to feed him frozen mice that way you both will be happy. They're pretty harmless so you have nothing to worry about.