I've done it too.
You need to work out first off who pays if the thing is damaged (driver or 50/50, and then what happens if one hasn't got the cash to do it?), and what happens in case one of you wants out. Bearing in mind the cost of clams on the Elise, even a slight brush with the scenery is going to cost a bit. Just make sure you get everything in writing and signed by both parties. Wear and tear on tyres, brakes, suspension etc, all stuff to consider.
Has he ever driven an Elise before? They can be a bit twitchy if you're not used to them, and that makes those car to scenery incidents a bit more likely to happen.