huh? No he is wrong. Yes, there can be a slightly higher heat buildip by going with -1 front tooth, because it's a slightly tighter turn radius, but breaking your chain? I dont think so
I run a 16 tooth front, stock rear, and I've been running it for about 4,000 miles. I clean my chain with WD-40, and lube with PJ-1 Chain cleaner, and it's in great shape.
With 300+ horsepower bikes, yes the tighter chain radius can be a problem. But on a semi-stock bike, -1 tooth is a good, cheap upgrade to do. Thousands of people go -1 in the front, and it's fine. Tell him its your bike, and thats what you want to do.
Better yet, find a new mechanic