I suffer from osteoarthritis in my fingers and thumbs. My left thumb is particularly bad, and the top joint locks. Unfortunately, I'm left handed.
I have had strong NSAIDS which did help, but I could only have them for one month, due to the risk of gastric bleeding.
I had a hydrocortisone injection 💉 into the middle joint of my left thumb nearly three weeks ago. At first there was an improvement, but now it is very painful again. I have been told I may need surgery. My practitioner is ringing me in three weeks time to see if the injection worked. All very disappointing.
Sorry I couldn't be more positive. Hope your fiance has better luck finding a solution for her painful thumb. I have a thumb splint, but it gets in the way of everyday activities, and is awkward to get on and off. Sorry.

I also have pain nearly everywhere, but I have been getting injections in both thumbs for years. At first they helped for about 6 months, but now my left thumb has become so painful that I can't safely hold anything. I am having surgery this coming Thursday (10/13/2022) and although I am dreading it I have a wonderful orthopedist that I trust completely. He replaced my right hip last November and I went from horrible pain and being barely able to walk, to having no pain whatsoever. It's hard to believe the difference! I have absolutely no pain and can walk normally. I will write after the surgery and depending on how everything turns out, it might be an option for you.
Blessings and love, Beth