the only songs that have heavy sections and all clean vocals are: Face of Melinda which is all soft except the end, and A Fair Judgment, which is also on the lighter side compared to their usual heavy songs, but has a very heavy ending
their album with the least growling (besides Damnation) is probably Ghost Reveries
tell your friends to open their minds a little more, not worry about "heavy" and "soft" because if they have any actual interest in music whatsoever they'll eventually learn to appreciate both, buy Ghost Reveries, and pay attention to the music, the clean singing, and the growling
being interested in music is the big thing, if they're listening to Fall Out Boy and the latest Lil John collaboration, don't even pursue your objective
if they're listening to anything even remotely underground, then go for it, because that's a sign of potentially enough interest in music outside of MTV and radio stations that they may actually "get" it