You already appear to be melding different elements of symbology, so I am unsure what you're struggling with.
Satanists require no sanction from deities or governmental authorities to choose whom they love. For this reason any ceremony would be highly personalised.
Our High Priest Peter H. Gilmore does outline the basic format of a Satanic wedding ceremony in his book, The Satanic Scriptures.
I wouldn't post the whole thing up here but as an idea that you could blend in, you could begin by calling upon some of the Infernal Names associated with lust - Baphomet, Moloch, Ishtar, Bast, Pan, Mammon.
Satanists' do not necessarily make their vows "forever" but rather for as long as happiness prevails.
Your fairy-faith (whatever that is!?!) friend may not like that aspect of a Satanic ceremony. So I'd leave any symbology alluding to that out, if I were you.
I mean really, no one can advise you accurately. It's down to the two individuals.
I would say it's just a case of merging symbology, doesn't your Satanist groom friend have any stipulations he'd like to see implemented? Can't you ask him?
Human beings are as significant as a cigarette burn in the sun.