I've had rotator cuff surgery on both shoulders at different times. The right shoulder was especially bad. They had to drill holes in the bones of the upper arm and shoulder and use laces to hold it all together. Took nearly a year until I was back up to par. The left wasn't as bad, but still took awhile. Physical therapy is a must.
I'm also right-handed. Here's the bad news: Tell her to start RIGHT NOW practicing using her left hand. She will not be able to use the right for awhile. Tell her to start brushing her teeth with her left hand. I had toothpaste all over my face for awhile trying to get used to that!

Also... Don't know a delicate way to say this, but tell her to also practice using toilet paper with her left hand. Those were some things nobody warned me about. If she practices before the surgery, it'll help her recovery to be less difficult.
Both of my shoulder surgeries were quite awhile ago, so I don't remember exactly how long I had to wear a sling.