its most likely one of two dilemas. ither the muscle is just to stiff in which case a ten or twenty minute session with a tennis ball each morning may do the trick after a couple of weeks of steady application. she simplely lays on her stomach and place the tennis ball between the points of concern and the floor, then rolls, breaths into it, lays wieght on it, lifts the leg while in postion etc. just have her play with the possibilities as her body urges and dictates. if its over use, particularly in the pose then she needs to find a way to shed the light of her awarness on this. the best and most direct way is biofeedback. otherwise a very skillful use of mula bandha and smooth steady breath may help but not necessarily, this method requires a very patcient and curious personality. or point her in the direction of a good bodyworker who can help her isolate the over activity and release it. a really good pilates person can be helpful here.

good luck

Josh,

I need a little more information: which forward bends, standing or sitting?
Hip flexors include about a dozen muscles. Please, be more specific about the location.
Also, describe the pain: sharp like a muscle pull, or cramping from binding?

If the student is very flexible, it could be that she is not creating enough space across
her hips before the forward fold. First spin the inner thighs back and apart, then scoop
the tailbone. Don't loose the rooting of the tailbone during the forward fold.

That last action also keeps the front hip bones (iliac crest) lifted away from the femurs
even in deep forward folds.

I hope that helps. Let me know what more you learn.
Blessings,
Scott