Putting your foot down won't help in the slightest.
Your stepdaughter is young and single minded. This man will be playing on her emotions - you're the only one I can rely on; you're the only one that understands - everyone else is against me; it is so beautiful you can see the real me; I really love you, you know I could never hurt anyone; I promise.....; how come you can see the real me when everyone else can't....
Turning against him without meeting him will only prove to her that he is right - the whole world is against the man she loves (or believes she does).
Seeing him in her home environment may open her eyes to reality. Far better for you and her dad to know what you are up against and hopefully to subtly show him for what he is.
Too many hot headed 17 year old girls leave home for love because their parents are against the relationship, and it's not always the sort of girl you'd expect. It can be the quiet, shy, introvert girl who is flattered to be getting so much attention; it can the in your face extrovert who pretends she doesn't give a damn what you think.
I would suggest you welcome him in to your home, be friendly. Accompany your step daughter to 'support' him in court if the relationship is still going at that time. Let her hear all the evidence not accusations from you or her dad.
With a bit of luck he'll be history in a few weeks and she'll have moved on.
Good luck, you're between a rock, a hard place and a soft heart.