From what you have said, I think this is his way of making his help easy to accept.I told him I don't want him to do something like this for me but he said it would be a business investment for him.
Ask him the question. See if he is willing to bear that risk. If I had this deal with a friend I would work twice as hard to make it work. It may be hard to sell prints, or it may not, but you never know.What if he pays to have all these different items printed & done up and they don't sell!
The two of you need to agree on this before you start. I would think if it were a real investment he would want half. I think you will find out how much of the offer is friendship and how much is business from the response you get from him on this one.And if (and that's a big IF) any of them do, what percentage of the profits would be a fair amount to give him for doing all of this for me?
This is exactly why you should do it. Think of it this way, photography is still a hobby. You have just extended your hobby to another level. If you work all day selling prints, make enough to pay your friend back and then make 75 cents an hour for your time, thats just a little bonus back from a hobby you enjoy. You go home with the satisfaction that the prints you sold are on someones wall now. If you really make anything it is a big bonus.I don't know if I should do this! Since I started photography as a hobby
Last bit of advice. He is a friend, you are right that business relationships destroy friendships. Talk this over in detail with him, and make sure you will be comfortable with it. A friend that would help you like this, is not a friendsip that you would want to destroy over a bad deal.