If recent injury (last 6 months) or post~surgery, rely on PT or Ortho for guidance of the clients program and clearances.
If previous injury: Know if they are “coping” with torn PCL, or plan on surgery. This would affect training modalities and goals. MCL partial tear is usually self-healing but can be painful depending on type.
If just looking to help with inflexibility, I’d concentrate on SMFR, dynamic and static stretching programs for knee and hip flexion, extension, functional balance and core strength. Content would depend on the type of athlete or non-athlete.
Hope that helps!