You don't. Something doesn't add up.

As long as he had a somewhat decent job he should be able to get a loan for $7k, even if it's like 20% interest... which would be cheaper than yours.

If he's that large of a risk to a lender (like doesn't have a job, is $100k in debt already on $50k income, etc) don't loan him the money. Also stay out of it if he's doing something illegal, like selling drugs which is why you know he'll be able to pay you back, but a bank or other lender wouldn't approve that...

Something doesn't add up, and you're charging him WAY to much.