Edited to add:
My Mom has MS and I know what your friend is going through. With something like that, it's not really the lack of faith in the horse's training, but the fact that the rider has very limited strength. Even a well trained horses will sometimes need a little bit more pressure to remind them that they really do
need to listen....and if you simply don't have the strength to give enough pressure in the snaffle, then a leverage bit really is safer.
There is almost always some initial confusion when a horse is first changed from a snaffle to a curb. What I suggest, since the mare has only basic neck reining skills, is to spend some more time working on that. Get her to where she's neck reining really well before trying to move to the curb.
Then, choose a curb with a solid or barreled mouth. The thing about using a broken mouth curb bit like a tom thumb or a dogbone bit on a horse that doesn't neck rein very well is that when you pick up one rein as a direct rein reminder, it causes the bit to collapse into both sides of their face and can cause some real confusion.
I would advise something more like one of these, this is the type that I use and have always had good results with this style. Saddles Tack Horse Supplies - ChickSaddlery.com Reiner WIde Port Swivel Shank Bit Saddles Tack Horse Supplies - ChickSaddlery.com Francois Gauthier Antique Hinged Futurity Bit Saddles Tack Horse Supplies - ChickSaddlery.com Kelly Copper Inlay Arch Mouth Bit - 5" 239091- Partrade Cowboy Collection Short Slotted Futurity Bit
What I do for the first few minutes of the first ride in a curb bit is I will generally take up light pressure on one rein, just enough to get them to feel it, and pull it out away from my body in a very exaggerated direct reining motion. There is usually some resistance and confusion at first but I just keep the light pressure until they figure out the new pressure points and leverage and give to that side. When they do, I immediately release all pressure and give them a scratch and then start again. I do that to each side until they are comfortable giving each way to the new bit. I do all that before I ever ask for any type of forward motion. Sometimes it takes just a couple of minutes, sometimes it takes 15 or more but they all get it eventually.
Then, just ride her like you expect her to ride. Until she has some more time under her belt with neck reining, you may have to give a direct rein reminder occasionally, but with a bit that won't collapse on her face, that is no big deal.