Great question. Based on the information you are providing, it sounds like they have an upper cross syndrome. The traps, sternocleidomastoid and levator scapulae need to be stretched and a good integrated exercise to help strengthen the weak muscles is doing a floor cobra. I would recommend doing an overhead squat assessment to determine what muscles are weak and what other muscles are tight prior to working with a client. Starting an exercise program without identifying potential muscle imbalances may lead to the cumulative injury cycle.
Upper Crossed Syndrome is a great place to start and my be the reason. If the person is already fit then I would look at the current program and see if the horizontal and vertical push to pull ratios are even. If not then I would start from scratch by activating the rhomboids, mid to lower traps, lats and rotator cuff muscles to strengthen the back side. I also make sure to shoe my client how to set thier shoulders and make sure that they hole that position in certain movements to train the correct musculature. Also Mackenzie stretches, chest stretches and hip flexor/abdominal stretches my help with the posture.
I would ask these clients to stand against a wall and place their shoulder blades on it, I would possibly as my PT to show me how to tape thier blades down and back and have them walk around for a day with the tape to let them experience what we are asking. I would attempt to get them on a foam roller and open up their chest, sometimes this is impossible for those with really rounded shoulders, I would bring my anatomy book and show them the shoulder girdle, I would tape a coin in between their blades and ask them to touch it with thier shoulder blades…..
I would take a picture of them standing sideways: I would ask them to not sit for longer than 20 minutes at a time, I would ask them that instead of pulling your shoulders down, lift your rib cage up…