I've got a similar problem, (mainly on my left hand) and am now thinking about the DCLs as they can be had fairly cheaply these days. I tried lefthand Sram X0 gripshift which does reduce the issue with thumb shifting but puts more strain on the wrist, which are two of my most affected areas. I had a little play around with having both shifers, (1 grip and one thumb), on the righthand side but couldn't get it to sit in a comfortable/usable position.
I have little problem with braking, although I can't 1 finger brake consistently on the left.
Without meds I couldn't ride at all and have learnt to adapt to my condition – I wouldn't contemplate not riding at all unless I got to the stage where I couldn't ride safely. I imagine your friend is fairly similar situation. Personally I find that I can't ride 2 days in a row without aggravating things further but I can live with this.
Let me know how your friend gets on and if they find any alternative options.
*EDIT – Forget to say that although the DCLs are fairly cheap, you need to switch to a full shimano braking system which obviously bumps up the price and may not be what your friend wants.