Make sure you have a good number of actual customers who say that yes, they would purchase this, and at what price. You can't assume that just because you think something is a good idea that other people "would definitely" think so too and want to pay for it. Samples are risky because there's no shortage of consumers who will show an interest in something as long as it's free.
With that out of the way, if I were your friend and determined to introduce my own line, I would want a small quantity for the business's use to include in a service package where I can charge for time and not just a markup on the hard product. Then, if that package were popular, I might consider making the product available for retail sale. This also delays issues like packaging until there's a good reason to resolve them and some money to spend on it.
On the other hand, there seems to be enough manufacturers already that he can just buy the stuff wholesale and have it on the shelves in a week.
Now, if you two go ahead as originally planned, then you should give him a price for the cards (and whether that includes printing or just the design), and the samples after researching your total costs in producing and packaging them. It goes on his books as a professional fee and you report the income on your personal tax return in the spring. Or he can hire you temporarily, have the business cover the hard costs, and pay you for the soft services like design and getting the product into little bottles.