Lost or stolen playstation 3?

My friend left his ps3 at my moms house, where i used to live. I moved out about a month ago and when he went back to the house his ps3 was gone. We think that my moms boyfriends kid might have stolen it but arent sure, it may have just been misplaced for some reason. Now my friend is threatening to sue my mom because he says that she was responsable for the ps3 because it was at her house. I dont think my mom should have to buy him a new ps3 because he left his at her house and it got stolen. But if it comes down to it. What should she do? and is she responsible for the ps3?