I am so sorry to hear about your wife's accident. Thank goodness she was not more seriously hurt. I had a cinch come loose on me and when the horse spooked, the saddle went off the side and so did I. (not hurt and no big problems here, thanks to my good horse)
I dont' think it had anything to do with the traditional western cinch tie, as I have used that for years on only when I didn't tighten it properly did I have trouble. I also had a dressage saddle slip on my once, so again, it was MY error in not tightening and rechecking at tacking up.
I think it should be easier for you wife to get back into the saddle than for some who suffer scary injury accidents because in this case, it is a preventable scenario. This need never happen again as long as she does the tacking up correctly. If is were a freak accident, or an issue of the horse misbehaving like bucking or rearing, then one can see that the rider could retain a lot of fear when getting back in the saddle that the whole thing will happen again. But, in your wife's case, she need not fear that because that one small thing IS in her control (as opposed to so much that we deal with when riding that isn't really within our control)
I suppose her doctor will tell her when it's ok to get back in the saddle. But hopefully, she will be able to be around the hroses and maybe do mild ground work so that she stays emotionally connected to them and more ready for the time when it's ok to climb aboard.