L.C.
How considerate of you. My best friend had a mastectomy; she didn't have a lot of pain but I would think that pain would be an individual thing so some people might have a lot, some not so much. I do remember she had limited mobility with her arm at first as some tissue had been removed under her arm.
I'm not great with gifts but some sort of relaxing foot bath or lotion might be nice. Inspirational books or books and movies to make her laugh will put her in a positive frame of mind.