Dark fetish? :-/
Sex line operators working for a company are trained not to discuss incest, bestiality, necrophilia, paedophilia, and scat fetishes (although a freelancer would probably just take things like scat fetishes in their stride). Actual threats that are not part of BDSM are obviously a no-go area.
Other than that I'm not really sure what you would class as a "dark fetish". I've heard it all, from vorarephilia to...well...I've heard it all. I can't imagine what your "friend" could have said. As for being reported, it sounds like the sex worker was from a company rather than freelancing, so "your friend" will just be barred from calling the sex line again. They're very very unlikely to be sending police round, it's quite well known that some people just prank sex line operators and say all kinds of stupid stuff, just to troll.