I would pretty much stay away from lower body exercises completely until that improved. I don’t mess around with injuries unless they’re my own. Hamstring strains can linger on forever, especially if you keep aggravating it. Light stretching is probably the most I would do – of course that depends on the severity of the injury. I wouldn’t do anything that causes the slightest discomfort to that area – let pain be the guide. If it lingers, your client should have it evaluated by a medical professional if they haven’t already done so. I hope your client feels better soon.