Its true that frozen shoulder takes long time but I would like to focus on major issue where most of elderly patient with shoulder case are diagnosed as frozen shoulder. And we miss the main cause. And even that can be a reason where PT session was not helpful .
Next Steps
Visit again to the orthopaedics or physiotherapist.
Get your mom's shoulder completely assessed.
Health Tips
Till then you can apply moist heat pack ( no dry heat pack).
Avoid painful movement ( if it is tendon or muscle injury, vigorous movement can leads to major problem)
Ask her to do Mild exercises involving bilateral movement.