so... my friend INSISTS that troll horns have some weirdly sexual purpose, and keeps claiming "the whole fandom knows this". is there ANY evidence or implication for this in canon or is he full of shit?

That’s fanon. I’m pretty sure that got started by the hordes of Hetalia fans that hopped onto the Homestuck ship back in the day since Erogeonous Zones are kinda a thing in that series.