Cousin hasnt worked out in many years but trainer still worked him too hard and he developed rhabdomyolysis. shouldnt the trainer have known better?

Yes, he should have.: Some trainers are task masters, which is perfectly fine if the person they are training has been conditioned to a high level of fitness. I always recommend that when someone starts a new athletic program, it is extremely important to start easy and work up gradually. Very sorry this happened to your cousin. Send the medical bills to the trainer.