Any exercise which is doable at their level, with leg extensor muscles (from just the movement through the elastic-based exercises-to the cable machine, for example). Also, if they can, use the steps of various heights in order to imitate the movement. Squats, sitting down and getting up and lunges, if they are OK for your client, would help. good luck! 🙂
is this client new to you? Or have you been training for a while, and this is a new development?
In addition to the obvious answer of checking with a doctor and/or PT, I would assess or re-assess and look for compensation there. I would first look for knee and hip extensor weakness but do not limit yourself but look at the entire chain.