Is the baby in child care? If she is and is crying like that every day, then your roommate needs to talk to the director and the caregivers there but I would be concerned if she is crying that much and they haven't already approached the subject with her.
There may be more going on here than separation anxiety.
But, if the mother has her in child care all week and then leaves her with you on the weekends, it could be a simple case of baby needing more time with mom. She associates her mother leaving with the mother being gone all day or its possible that she is associating being left with something that is going on in her normal child care.
The mother needs to look at all possible reasons for this crying. Take her to the doctor to be evaluated, talk to the child care providers, etc.
And, honestly, because you have been leaving her in the bed to cry, its very possible that is the association this child is making with her mother leaving and is only making the situation worse.