It's more a subgenre. Most romance publishers will ask for BDSM-themed romance stories. it's a lifestyle choice, and publishers like to see how the dynamics play out with or without the kink. It doesn't have to be erotic; it can be erotic romance/romance with any other sub-genre. My one series is BDSM psychological Thriller, dark romance.
There's a mass of BDSM, with most romance publishers eg.
here. With your friend. I'd definitely recommend picking up a few BDSM novels from whichever publisher she wants to submit to. That will be a good guider for how 'light/dark' that publisher likes with their BDSM. You can also mention going to Goodreads and searching up BDSM romance novels there. That will bring up multiple novels by different publishers, and they can trace publishers that way.
As for marketing? The publishing company will know how to handle targetting a BDSM readership. But beyond that, it's much like anything else: knowing your target audience. Again, Goodreads is good there because they have BDSM groups for readers and authors who like BDSM. You'll also get lifestylers who are looking for BDSM novels. But also talk to other BDSM authors. Like with the MM community, they're a great bunch of people. I sat on a BDSM panel at an author signing event two years back: both are authors, one a Dom who trains Doms, and I think it was brilliant. I'm not a lifestyler, but I'll use a BDSM trainer who is when it comes to conceptual guidance on the lifestyle as a whole. You can get it wrong (like with 50 Shades), and that's when it upset those who are BDSM.
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