Tight hamstrings would likely be the culprit, as opposed to tight hip flexors which would help keep the person sitting in that position.
You will want to pay close attention to the client and do not let them move past the point of discomfort in case it is due to an injury. Other muscles used to help lift could also be weak.
Therefore, figure out if this is due to poor flexibility, poor strength, or both, with open communication with the client. Good luck.