Hello Ellijah Mussig,
You want to check with their doctors for clearance. Then you want to train away from the injured areas but still pay attention to pain since the body is one unit. Less flexibility will amplify issues so you need to make sure the client and you both are patient and let the body tell you when it is safe to progess. Everyone is different and heals at their own rate. We all have a different tolerance for pain also.
I like to do flexibility training for full ROM before adding resistance and moderate cardio.
is this a recent injury or years old? There should have been physical therapy, and you can start by talking to the PT and progress from there (physician’s approval assumed).
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!