Lot's of good advice here folks. Thanks.
To be clear, when I invite someone out, I rarely accept contributions for fuel etc. as it was my invite after all. Contributions of food or hooch are readily and happily accepted.
In the case I raised, this chap is a good friend and has been a good friend for more than 20 years. We don't dive much together in recent years as we each had our own boats and "tenders" and each of us dove in our own way. Recently, he has separated and sold his boat. I am happy to have him come out with my partner and I on occassion, but I know that "occassionally" will rapidly become a more-or-less permanent situation. I suppose that now that I have said this out loud, I am realising that that is the greater issue. I don't want him there all the time... and perhaps insisting he pay his way is my way of discouraging him!
Man, this place is better than Dr. Phil!