Flexibility doesn't equal mobility, one is being able to be manually put into a range of motion, the other is to be able to control joints at end ranges of motion.
Hip flexors are a group of muscles. I'd be a little skeptical and want another opinion if a practitioner said just to rest. In my and my kid's experience, complete rest has almost never been the answer.
Francis, depending where you are in NJ, I'd highly recommend you check out Challenger Strength.