Severe pain while doing physiotherapy after knee surgery

My mother has had spontaneous knee dislocation for most of her life. Usually it would move back on its own, but the last time, it did not, and the damage was severe enough that surgery had to be done. She was on bed rest for a month before surgery as they needed the swelling to go down first. About a month after surgery she began physiotherapy. She has been making slow progress, slower than other patients of the physiotherapist. At the last session, something snapped and caused her an immense amount of pain. She hasn't been able to move her leg for the last two days.

She seems to be really slow in her progress, and I want to know if it's okay to continue pushing past her limits, or if we should ask to go slower. My mother didn't really understand what exactly snapped in her leg, so if anyone has any idea, please let me know. She's been doing physiotherapy for two months now, and the physiotherapist was able to get her leg to bend to 105 degrees. She was able to bend it without any pain on her own to about 95 degrees. Is this abnormally slow progress?