You could try regressing the press up's for a while until your client achieves better scapula control. Try getting your client to set his/her scapula and hold it in a retracted position whilst performing a press up on their knees or if they cannot manage this, try some in standing against a wall. With regard to corrective exercises for scapula control keep the wieght light and the movements quite slow performing lots of reps with the scapula set in a neutral position. Also regular stretching around the chest may help especially if your clients shoulders are protracted.