You have said it's a well written novella DG, so it's hardly 50 Shades of Grey (anyone recommending that particular piece of nonsense goes down in my estimation!) I'm sure we all remember the ridiculous furore over Lady Chatterly's Lover, which although written by one of the 20th century's most talented writers, and that novel must seem risible to today's generation.
My own mother was too inhibited to even talk about the court case... and as for Oscar Wilde
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There has been a huge generational shift in talking about sexual mores, and all to the good IMO. I'm sure your GD has no such inhibitions, so please..... do as Ninathenana suggests and if you are really worried about this flippin' book, hand it over with a light-hearted remark such as 'the opening's a bit raunchy, explicit, near the knuckle' (strike out whichever you feel uncomfortable with).
I feel you are casting your very good friend in an extremely bad light here. Do you have doubts that he has some dark side? You need to think about this... if so why are you in a close relationship with him? I'm sorry but the problem seems to be of your making, and only you can solve it.