No reason she cant reach the GP level of the sport. Its the top rung of the ladder and OK as an EVENTUAL goal.
The best way to mentor this teen is to center her ( and her parents) aspirations on the rungs of the ladder needing to be climbed in order to reach the top. Right now, focus needs to be on moving up from crossrails to 2’ then 2’6” managing pace, straightness, basic distances, lead changes and spreads/oxers then up to 3’ with more complicated course questions and fences that start requiring more precision, control and are less forgiving of mistakes.
Each level adds more control, strength and balance which must be mastered before adding more challenges for the safety of both horse and rider. Both Hunter and Jumper require this basic foundation up to this level.
It will be highly unlikely one horse and, probably, one trainer will be able to advance her to this level and certainly past it to specialize in moving up the Jumper levels. If shes serious and dedicated, just a guess, 3-5 years from crossrails to being competent and competitive in, say, 2.5 to 3m Jumpers. Then she can start looking at moving to the real National level GPs.
Of course, there are a million variables including individual talent, desire, ambition, willingness to really work at, financing, injury and fear, burn out…all sorts of things. Shes still a kid now, who knows, sometimes they turn out scared to jump higher ir fall in love with a step up horse and it breaks their hearts to choose between keeping it or moving up so they decide to stay on the hobby level. Parents and trainer(s) need to be prepared for this. And, horses break. Its a tough sport.
Anyway, above is kind of a map. For right mow, I would honestly try to get her and parents to set a goal of becoming competent and competitive in Childrens Jumpers before investing heavily in anything GP level related. Seen a number of teens reach that point in 2-3 years. Encourage her and parents to focus her dreams on that Child/Adult Jumper level.
Be remiss not to note many parents are satisfying own lofty ambitions and suck the joy of the sport right out of the kid leading to burn out and guilt for “ wasting money”. Often kids just want to ride and socialize, not seriously compete…and thats FINE.
Trainers can take advantage of less knowledgeable parents by pushing expensive prospects that will require many years and $$$$$$ in training fees to develop to elite levels while kid needs a schoolmaster to ride, also adding to trainers income.
With all this in mind, they need to focus on the lower rungs for now.
Ummm, just a thought, have these people ever seen a real GP? Do they have any idea what it takes to get around huge courses within the time allowed? How much strength and skill from the rider? How hard the horse pushes off and lands and those sounds? Seen a rider get stopped off into off solid wall or a horse go down into a big spread?
Make sure they know the reality under all that pixie dust of fame and fortune.