Investing in Amazon Business Suggestions

I've known this small business owner for a couple years. He's got a small, profitable health supplement business which I believe in and would take myself. Over the course of a year or so I've casually asked a couple of times what kind of deal would look like for investors if he ever wanted to expand. He always said the business isn't ready for investors. I never told him I wanted to invest, the topic just came up a couple times when we were talking business.

Anyway, then the other day he mentioned he's considering asking the boss at his job to invest in the Amazon business. So I told him I may be interested in investing and what would a potential deal look like. He doesn't have a proposal or anything ready to go, but the basics look like this:

-$40,000 - $50,000 needed for product production
-After expenses, expected profit margin is about 40%
-Base case for return: sell thru in about 1 year or less;
Worst case: 0 sales but he said that's highly unlikely as he's got an established brand with proven sales in an adjacent market, is pretty good at Amazon SEO, etc;
-best case the product sells through in 6 months or sooner and we keep re-investing & expanding

I told him I wouldn't put up the whole amount, but would be interested in putting up a portion of the money and be on equivalent terms with the other potential investor.

How would you structure this and what would be your investing parameters?