yes the enclosure should have been locked, especially when youre staying with a roommate. the key should be kept to yourself unless youre going to need your roommate to feed your snake and stuff when youre away.
when you say that the mouse had been around the sick snake for days, are you saying that they left it roaming around the snake??!!! you deffinitely need to monitor your BP`s health closely for the next few weeks. and yes i would be pissed.