I think the only way to find out if your friend can do aikido or if it helps her arthritis is going into a dojo and try.
We have a girl training with us who has arthritis, she trains regularly and is a 4th kyu. There are things she doesn't do (yet) and we respect that. Everyone has his/her limits, you have to accept that and work on it. Maybe next week someone is ready to cross that limit, maybe it takes a year, maybe it will never happen. That doesn't mean we cannot work with each other, we just go sofar that both of us are comfortable with the exercise. I work with her regularly, even did my 3rd kyu exam with her. However, I am always careful with her and make sure she doesn't has to breakfall too roughly but that is more on account of the fact that I am over a foot taller and am 50 pounds heavier
.By the way we train aikikai-style and regularly attend national seminars, arthritis hasn't stopped her from coming along. So bring your friend with you to a class and see what happens. After all, your friend is the only one who can decide if aikido is something for her or not.