If she's "always at the gym" then she could be potentially OVERtraining, and not giving her body the recovery not needs to grow.
Flat out NO to hormonal use! She can easily get her legs bigger if she gets her DIET right and trains for it. Considering the hormonal route at that size and if she's not really training for anything is flat out stupid.
FOOD is going to be her BIGGEST help in getting her legs bigger. If she's NOT eating enough, then she's NOT going to grow, period! And if she's always 8% BF, then she's probably isn't eating enough, since for a female to stay at that BF% they generally barely eat anything.
Supplements, she should be using are the staples: Creatine, multivitamin, good fats, protein powder, and BCAAs. She could also consider using a natural testosterone/GH booster.
As for training, squats and deadlifts are the best exercises to be doing. Try different variations of squats: Front Squat - narrow and wide stance, and back squats - narrow and wide stance. She should be lifting as HEAVY as she can on EVERY set (with good form).
I would recommend perhaps doing a 5x5, 6x6, or 10x3 programme, where she's doing Monday, Wednesday, and Friday. On each day do Front Squats, Deadlifts, and Back Squats. She can do a couple of accessory exercises for her upper body afterwards if she wants to, or do an upper body session another day.
Cycling is actually a REALLY good thing for getting great legs. I am an ex-cyclist. Granted my legs have always been big, even before i was a cyclist, but now, even doing 20-30 mins a few times a week still builds them up (even though I do NOTHING for them in the gym).