I don’t have arthritis (I don’t think - waiting on scan results) but I do get pelvic pain and struggle in certain positions during sex. I find that anything where my legs are too bent and taking most of my weight is really uncomfortable so cowgirl is reserved for times when I’m feeling better and only for short times.
I’ll have a look at the position of the week thread and see if I can find any of the ones that I like but it might take me a while. This is the thread that has all the positions of the week in it, it would be worth having a look through to see if anything stands out to you. It’s hard to say what positions would work best for you because it depends which part of the hips are affected, we are all different and it will be a case of trying things out.
There is another thread called disability fun which has some tips that might help. You could try putting pillows under her hips or consider getting a position wedge like this one: