Donald,
There are lots of costs other than gas and launch fees that most people don't realize unless they they their own boat. The time it takes to prepare a boat for a day's fishing trip; the cost in time and money in the clean-up after a trip, and so on. The maintenance costs., etc.
The decent thing to do is to genuinely offer to share the costs. If he/she declines, then that should be the end of the monetary side. You can do other things to make up for it. Helping with the"duties" on the boat such as managing the anchor and the anchor line; keeping your gear neatly-stowed; properly handling any rods or tackle he provides; goes a long way to showing your host how appreciative you are of his invite, and would probably open the door for other invites.
I own my own boat, and I fish with other people who own theirs. We all share the expenses, regardless of who's boat we happen to be on. And we all bring non-boaters to fish. Most offer to help defray the expenses. Some don't. Not the end of the world.