I do not think that he'd be participating in her fetish any more than someone who was asked to teach someone how to maintain their vibrator.
For one thing, if you're in the business of vibrator maintenance, you've signed up to be adjacent to people's sex lives in that capacity, whereas Elliot has not.
What does "adjacent to people's sex lives" even mean?
Does, say, baking a wedding cake for a couple about to be married make you "adjacent to their sex lives?" Apparently, some do. I disagree with them.
For another, a vibrator is typically just a tool of sexual stimulation, not itself the object of a fetish.
I suspected after I posted that someone would say that, even though it could well be the object of a fetish just the same as anything else would. Being an explicit sexual object doesn't prohibit it from being used as the object of some fetish. Regardless, substitute whichever mundane everyday object you like, if you prefer. It doesn't change my argument.
Anything anyone has ever manufactured could be a fetish for someone. There is no ethical issue.
I'm surprised no-one has brought up an ethical issue with Roko simply walking into the bakery and ordering loaves of bread without disclosing to the baker the purpose for which they are being purchased. Because the issue is the same.
I think I could buy a pair of boots and not be squicked if I found out later that the bootmaker had a boot kink.
If you are made squeamish by the thought that something you do in public might turn someone on, then walking out your front door could prove to be an insurmountable challenge.