Elliptical trainer and tight hip flexors
I don’t see anything wrong with that, it’s better than something like a recumbent bike or rowing machine (because of the standing upright position on an elliptical). Teach the client about movements like posterior and anterior pelvic tilting. What I like to do as a dynamic stretch for my hip flexors is put a treadmill on an incline and slow walking speed, actively keep my hips in a posteriorly tilted position, and take very long strides. This has helped me immensely; I’d recommend trying it & telling your client about it. Also teach them hip flexor stretches like the runners stretch. Good luck