The fact of the matter is this dog is her responsibility not her room mates! Yes the room mate should know alot better but the point is you obviously have had it happen on other occasions, there is a risk of it happening again so who's problem is it? If she thinks her dog is in danger then she should move out or kick the roommate out.
If My roomate left a bag of drugs on his bedside table and my kids ate it and had to go to hospital, nearly dying. There is no way in hell that I would chance it again, I would move out! Simple as that!
Once its happened time and time again the fault then stands with the owner of the pets in my opinion. The same as If I lived with a drugo and put my children at that risk, Its my problem!