Given what has happened to her mother, it's not altogether surprising that Sal has never mentioned her to Phoebe or anyone else, for that matter. Sal's mother ran off to Idaho after a tragic miscarriage and a subsequent bout of depression that she just couldn't handle. This is not the kind of thing that people normally tell their friends, and Sal is no different in this regard.
Sal has only mentioned her mother to Phoebe once; that was when Phoebe asked about her and Sal said that she didn't live with her family. Phoebe didn't pry any further, maybe because her own mother had left and because she didn't want to get into a conversation about such a delicate subject.
In any case, Ben is now being pretty nosey. He says that he's seen Sal's father but never her mother. He comes right out and asks Sal where she is. In reply, all Sal can do is tell him that she's in Lewiston, Idaho. When Ben asks what her mother's doing there, Sal refuses to spill the beans. She merely confines herself to saying "I don't really feel like saying." It never occurs to Sal to ask Ben where his mother happens to be.