I know it is poor form to answer questions with questions, but posit...
Who gets to pay for remedial work if the County Building Inspector rejects the work?
Who is signing off on the project as the Professional, Qualified and Expert Engineer?
Who gets to pay if the resultant owner of the deck is a member of the 350+ club and has thirty like-friends over for a BBQ and the deck collapses injuring a bunch of people?
Why is it called "Safety" factor?
Does meeting the contractor's demands jibe well with your perception of your own integrity and professionalism? You need to start out as you intend to finish.
Have you contacted the Building and Zoning department and asked them what they think about handing out a variance? You may want to mention the contractor's name so the Inspectors can keep an eye on his shoddy work. It is the end customer that will pay the cost of remedial work after the warranty runs out.
Having started, run and sold a few successful businesses, I was given a Platinum-standard piece of advice in the very early days of my first business. "It is far better to send those clients to your very next and biggest competition. Sooner or later it will bite back and rather them than you."
Making a fast buck on the promise of something is short sighted as I suspect that your client will go with whoever will gives them the lowest price for the next six decks. So my suggestion is, make your decision based on facts not on what might be.
Just saying, ya know!