if your mechanic let you go 100,000 miles without suggesting to replace your serpentine belt, then you should find another mechanic. He either didn’t look, or doesn’t care. Neither are good on the trust scale.
If you didn’t listen when your mechanic told you that your serpentine belt was going to need to be replaced soon, then you goofed- and should buy them a cup of coffee when you have your car towed in.