Oooh, I used to work at Kings Langley Riding School 14 years ago (OMG - 14 yrs!). I believe it has changed hands since so I can't really comment on what it is like but I would definitely recommend if she wants a good hack, as there are fab bridlepaths around Chipperfield common.
Greinan in Bovingdon is very good, we used to go to Evening jumping their - the facilities were brill.
Rose Hall in Sarrat was also well regarded when I was in that area, and Patchetts was the most serious RS, although I think they may have stopped lessons at Patchetts.
There is also Lincolnsfield in Bushey - which is in a kind of outdoor activity centre. Quite a few of the liveries at Kings Langley had come from there and rode well, but again, no idea what it is like now.
I also went out on a few hacks at Woodley in Hemel, and did my school work experience there, when they only had 3 stables and it was very basic, but I know they built a whole new stable block and so that may well have improved.
I moved from the area 10 years ago, so much has probably changed, but I hope that gives you an idea. I really miss the area for having horses, I am in Devon now and bizarrely there were far more good hacking places around Hemel/Watford!