Hi - are you sure she is seeing a physio? 12-16 months MORE is unreasonable UNLESS she has "frozen shoulder".
Otherwise there is another diagnosis that has been missed.
It sounds like the physio and physician are guessing becauseMRI has not been mentioned by you. As physio.irl mentions, it is best to get an MRI to see what is going on in there.
To give you an idea, i expect my patients to get better in 3-5 treatments over the course of 6 weeks. If i can't make a change in that time, then there is something there i don't know about (needs further scans etc) or it is something i can't help - it needs surgery. There are of course conditions which are clearly outlined which need more time than that but if it is really a muscular issue, it will respond in that time. I have a personal aim of helping first visit patients achieve 70% improvement after the first treatment.
All that is just to say that i would almost never recommend someone come to see me for 12-16 months to get better...
Good luck!
Get MRI and another opinion from a surgeon or another physio...