What interest do you think is fair?
If you're lending to a friend that you trust, I'd suggest a low, "friendly" interest rate. Most lenders request an interest rate that will provide returns while also compensating for the occasional default. If you're confident that you'll get the funds back, I'd put generosity above profits in this situation.
Here's a list of lenders on the forum including the interest they charge -
https://bitcointalk.org/index.php?topic=5135395.0It would give you an idea of what users regularly charge here and how to structure yours. The list has not been updated recently, but it still contains relevant information.
Yeah, I need to update that thread. It's been a while and I know some of the rates are outdated. And, at least one of those lenders (he'hem) is no longer offering loans.