I heard some very good advice lately regarding taking in stray iguanas: You take what you can, and no more. I know it's hard, but you can't beat yourself up over not being able to take this one.
He sounds like an iguana that will be easier than most to adopt out, especially if he comes with a cage.
Are these people your friends? They sound like jerks. Personally, it makes me furious that instead of coming to you for help and advice, these people are basically threatening you because they know you have a soft spot for iguanas. It's manipulative and evil.
Don't feel helpless, there are things you can do without taking the iguana. Make it clear that dumping the iguana with you is not an option. Try to talk to these people and tell them what will happen to their iguana if they drop it off at the petstore. Then, walk them through the process of putting the iguana up for adoption on the web. (I'll cut and paste the links I know below.)
Are you a decent photographer? Volunteer to take a roll of pictures of the iguana and put the best one on disk or send it to them as a file. Adoption postings with good photos will always get a better response than those without.
Maybe you can show the wife how to feed the iguana with minimal contact. All he really needs is food, water and a clean cage until he gets adopted. But not with you.
GIS adoption board:
http://members2.boardhost.com/gisadoptions/
Scales and Tails:
http://www.scalesandtails.org/
Petfinder:
http://www.petfinder.com