P.R.
I've never heard of this before, but I could identify with what you described. I've just always described myself as a "fainter." Bump the elbow or knee, pass out; see my own blood, pass out; on stage, pass out; get overheated, pass out; hear about some gory disease, pass out.
As I've gotten older, not so much. Partly because I learned to recognize the triggers and sit down, or change focus, or control breathing, or all of the above. Now, if I get light-headed, I can usually prevent myself from fainting. So, I don't know how you would coach her, but I think over time she will learn how to recognize what's happening to her and either lie down (which is how I managed early on) when she feels it coming on (so at least if I fainted, I wouldn't hit the ground since I was already lying on it), and eventually slow the reaction so that sitting with head between her legs will stop her from fainting. Now, I usually concentrate on regulating my breath if I know I will be in a compromising situation, or as soon as I realize that I am, and I can stop the fainting.
Hope this helps!