It sounds like your brother used a leather cleaner/conditioner combination product because leather cleaners typically do not leave anything behind. You need to strip the leather of the product your brother used. I suggest Optimum Power Clean diluted at 3:1 or P21S Total Auto Wash, also diluted. Use microfiber towels and you may have to spend some time buffing the seats with the Optimum Power Clean to fully remove the existing stuff that made your seats yellow.