Id be sure to have it ALL in writing before agreeing to anything. I had to write a business plan the other day on schol and its had as far as the research and detail but wasnt all that bad. I'd get it in writing so that if you have questions you could refer to it and make sure he holds up to his end of the bargain and also to see how hes doing on his projections. A business plan will answer most of your questions.
Are you giveing a loan or buying a permanent piece of the company?