Hamstring...gluteal...and the extensor muscles in the back. They're all interconnected in a functioal sense with respect to pike flexibility.
A shotgun approach flexibility exercise......Have her sit in a pike position on the floor. Let her bend forward while you very gently apply pressure. She should reverse the motion when she gets to the point of feeling a stretch, gentle pressure should be provided as she moves upward. She should repeat this motion 15 times, and the pressure can be slightly increased with each repetition as she warms up the muscle. So take it easy on the first 4-5 reps.
This exercise will accomplish more than you'd believe if you do it properly. The don't list......Absolutely no pain, no rapid movements, no bouncing.
The do list......Take it slow! Consider the first few sessions as orientation and training exercises while you both learn how to communicate. Do a set of 15 reps, and then stretch for five minutes in a pike, a slightly straddled pike, and a moderately straddled pike. Repeat this 1-2 times. Only do the exercise, at first, on alternate days. If she wants to after 5 sessions, she can increase the frequency to 2 days on - 1 day off.