There could be all sorts of things that are out of our scope as trainers that could cause painful gait patterns but if there structural issues we can certainly assess and address with a corrective approach. Its important to have a systematic approach when confronting clients in pain.
Maybe try something like this:
1.verbal assessment–identify injury history, painful regions, repetitive movements, footwear, etc.
2.visual assessment–identify potential common distortion patterns at the foot, ankle, knee, hip and shoulder
3.movement assessment–a step up test and a wall lunge with rotation are excellent options
You may determine that the condition is out of your hands but at least you can speak a language to the proper medical authority if referring out.
hope this helps,