I'm sorry to hear this about your mother. We all wish her and you the very best.
As a breast cancer surviver myself, I'd suggest that you make her something that looks and feels like a soft baby pillow. This will keep her arms from bruising her freshly sutured skin under her arms. Trust me, this area is tender...
And like the others say, remember to always tell her how much you love her. Remember to mention little things she did for you as a child, this will thrill her that you did remember all she did for her kids.