Tbh the guy at the gym might have a point but not for the reason you think. Asking for 15% straight up puts your boss in a spot where he either says yes or watches you leave, and most managers will just let you walk out of pride. What works better is asking for stuff that does not hit the budget the same way, like extra vacation days, working from home a couple days, or a title bump that helps you get paid more at the next job. So he is not totally wrong, he is just bad at explaining it.