Just be honest with the guy.
You've watched his back, and kept him from making himself a Darwin Award winner so far, but that's where your responsibility ends now.
If he's a danger to himself when not supervised, you would be doing him no favors by glossing over that and letting him be seen as competent in an area that he is not, to someone else. I don't know what it's like in SA, but as far as I understand it, you cannot give somebody a bad reference - but you can refuse to give one.
Let him know where you stand on this and why.
One day, if he is still around, he might thank you for it.
My 0.02.