Black Lace doesn’t publish romance, though. If she wants a romance (which is mainly defined by a certain level of monogamy throughout and a happily ever after at the end), then Black Lace wouldn’t be what she’s looking for.
Definitely Bertrice Small.
NOT the Anne Rice “Beauty” books - they are hardcore S&M porn, not at ALL Romance.
Sorry, I know filthy, but not romance 
susan kearney and susan johnson are the most smutty. look for the “blaze” line of silhoutte books. they are super smut!
nearly forgot laurell k hamilton. oy! she has gotten quite out of hand.
I agree with the suggestion of Susan Johnson, and also suggest Rosemary Rogers.
Ellora’s Cave is a pretty good hump…errr jump… off point for erotica.
Erica Jong and Judy Blume; however The Fermata will blow your mind. Anais Nin is always good (but a bit much, even for me, at times!)
Wow, thanks everyone for the replies. I have a couple of questions:
Do you guys mean that those books are “erotica” and not “romance”? Are they the same thing, except romance is more focused on monogamy? Or is there a greater distinction than that? If that’s the only distinction, I don’t think she’ll care.
Thanks again everyone for all the great tips. Keep 'em coming.
I just ordered Skintight for my Kindle per your recommendation. I love a hot sex scene–good thing I can Kindle with my left hand! 
The romance genre has a few basic rules that keeps it distinct from other genres (including ones that are closely related to it like Women’s Fiction and erotica). Romance novels have two main plots, rather than a plot and a subplot. The relationship is the main focus of the book, but often, there’s some other sort of danger or intrigue going on that runs concurrent and is as important s the relationship. There are often complications and triangles, but it’s usually pretty clear who the main love interest is for the heroine. At the end, the two protagonists have declarations of love and the implication is happily-ever-after. In other words, in romance, it all works out in the end.
Erotic romance is as described, but the relationship is highly erotically charged, there’s a great deal of explicit sex between the protagonists, and often between the main characters and secondary characters. The secondary plot might revolve around sexual matters as well.
Erotica revolves around sex. The protagonist may sleep with multiple people, the themes may be darker, it may deal with subject matter that romance usually doesn’t, and there’s no requirement for a secondary plot (though there may be one) or a happily-ever-after (though there may be one of those, too.).
Then there’s commercial fiction with sex in it, which counts as neither romance or erotica, like Harold Robbins and Jackie Collins.
The increasing popularity of digital publishing has caused many of the distinctions to collapse. In fact, digital publishing completely changed the face of the romance genre. Ellora’s Cave was started because the New York publishers refused to publish sexually explicit material. Now the pendulum as swung the other way, and it’s far more difficult to find romance without the smut.
They aren’t erotica either. Erotica exists to be erotic. Harold Robbins writes (extremely zany) novels that happen to have more sex than average. Jackie Collins, kind of writes these (extremely dated, IMHO) tales of Women Making It In The Big Bad World And Overcoming Obstacles. Having some sex scenes doesn’t make something a romance novel. They’re just novelists that sometimes include sex scenes. A romance novel has a relationship and the emotional development of that relationship at the heart of its plotline. And a romance novel always, always ends by affirming that relationship. That’s what makes it a romance novel, and why a romance novel doesn’t have to have any sex.
A book with lots of sex that does not address and affirm the emotional relationship between the protagonists, is not a romance novel. It wouldn’t have to be specifically monogamous (others have noted romance novels with polyamorous themes) but what it can’t be is about a whole series of relationships that fail, or about sex that isn’t in the context of an emotional connection (making Erica Jong virtually the anti-romance novelist)
Harold Robbins is not what I would describe as “zany.” Are you confusing him with Tom Robbins?
Also true of The Story of O, mentioned up-thread.
Oh dang, you’re completely right. I am.
I second Shirley Ujest’s suggestion. Ellora’s Cave has trademarked the term “Romantica” and they say “Erotic romance is defined by us as: any work of literature that is both romantic and sexually explicit in nature. Within this genre, the main protagonists develop “in love” feelings for one another that culminate in a monogamous relationship.”
They have different levels of explicitness and also publish in different genres, like humorous, sci-fi/fantasy, interracial, BDSM, same-sex, whatever. As pepperlandgirl says, the quality of the writing and especially the copy editing varies a lot.
I’ve read only a couple, and they were too explicit for me. But that sound like just what the OP is looking for! 
I am not aware of any other online smutty book dealers. Or any reviews of this kind of book.
Anyone have any direction?
You can search by hotness level.
YAY!
Too whom do I send a bill too?
Another vote for the Outlander series. Jamie can swish his kilt around me any old time.
Vox by Nicholson Baker. It’s not romance per se, but it is a very realistic telephone conversation between a man and a woman who meet on a “party line”, and pair off on their own. Very hot. Also, The Fermata, same author.
As a teenager, I was once bored enough at my grandparents’ cabin to flip through a few of my gramma’s romance novels. Most were pretty much what I expected (cheestastic), I’ll never forget the Longarm series. Looking it up now, it was (is?) a western romance series written under the name Tabor Evans. Anyway, they were remarkably dirty, at least to me as a kid. Not just tons of sex, but anal sex, oral, the whole nine yards.