You join a gym (at which you do not work) and you see trainers not doing their jobs the way they are supposed to, or you see fellow customers performing exercises with poor, unsafe form. Do you cross the line and correct them (trainer and/or fellow customer)?
As far as the trainer with the client goes, I’d leave that alone! But if it is just another member doing it, I’d introduce myself and would politely ask if he/she would mind me making a suggestion. Remember, few people like to be told that the way they are doing something is wrong.