Due to a very physical job, I had a rotator cuff injury back in the mid 90's and eventually went for PT.
But the physio really didn't help much. IMO, I just thought physio was a cash grab. But I guess it does work for others.
I just took it easy by being very cautious of my movements and didn't raise my arms in a quick manner etc.
Eventually I created my own exercises which really helped, because I really wanted to get this thing to heal and end the dull ache misery.
But because of the pain that came from the motion of just lifting my arm(s) period, I really knew that I couldn't not lift and use my arm and have things seize up even more and/or end up with "Frozen Shoulder" syndrome, which is actually almost more painful, so I created my own exercises for my rotator cuff, and eventually for everything Shoulder related.

I would plant my hand on a wall either with my arm fully extended out towards the wall or at an angle.
I then would Crawl my fingers/hand Up the wall until it was as high as I could possibly make it crawl up over the head, and then I would kinda lean my body inward to my arm and kinda turn so as to add a little bit of pressure into the shoulder back area to kinda stretch things out.
And then, after a bit of the stretching of things, I would then Crawl my hand back Down the wall.
I started with a couple repetitions at first and eventually would do 5 or more because the more I'd do, it would actually make my arm kinda a heavy tired, so I'd just do what I felt was not gunna cause more harm.
But, if I did this a few times throughout the day, it really helped with the pain, thus why I believe I healed my own rotator cuff alot sooner than had I gone to PT.

And I know this hand crawling exercise sounds weird, but that's because it's really hard to explain exactly what and how I did it, but this is the jist of it, if you can imagine using your Fingers to Crawl your hand Up and Down the wall so that it raises for arm and shoulder.
But in doing this exercise with the Crawling motion of my hand going Up and Down the wall along with the careful leaning into it and kinda turning/moving the upper body so I could feel a little bit of pressure in the upper back and shoulder area, it truly helped me far more than any Physio Therapy (PT)!!

However and Unfortunately, Later on in life, I did end up with Frozen Shoulder syndrome, but on the opposite side.
Frozen Shoulder kinda feels like the Carpol Tunnel syndrome agonizing dull aching pain, but it's in your shoulder!
But I did these Hand Crawling exercises for that as well and it absolutely felt really good on the shoulder and arm and helped ease that Dull Achiness because with Frozen Shoulder, it almost makes your arm feel dead...

I know this isn't a "Medical" Cure, but I hope this helps your Mum like it truly did help me, especially with the pain and laggyness of time in trying to heal...