According to the M/T specs the 33x15x15 ET slick has a section width of 18.1" and a tread width of 14.8". The only street tire I could find on the M/T website that's listed as a 18.5 is a 31x18.5x15. They do list a 33x19.5x15 which has a 18" section width and 14" tread width. These are both measured on a 14" wide wheel. The section width is the widest part of the sidewall bulge. So according to M/T's specs, your buddy is correct and they will fit.