Your friend's future employer asking you for a reference is a "business transaction", and you should not mix your friendship with it. If you have been asked for your opinion on their technical skills, and you don't have enough data, then simply state that. You should not call your friend "the greatest programmer ever to walk upon this Earth" simply because he is your friend. A statement like the following should usually work:
John has been my friend since 2008, and he is a pretty nice guy. However, I haven't yet had the pleasure of working with him, so I am not in a position to comment on his technical skills. If you are interested to know about his personality, maybe I could be of help.
If I want a friend to give me a reference, then it is my responsibility to ensure that the friend has all the relevant information required to give me a good reference. It can never be the friend's responsibility to figure out what I do and how good I am at it. However, I would suggest you ask your friend to provide you the required data in case someone contacts you for reference in future.
That said, however, the future employer should probably not be asking you about your friend's technical skills in the first place, if they know that you haven't worked with him. They are much better off asking your friend's colleagues or manager.
Most companies ask the candidate for names of people who could be contacted for reference. Even if they don't, your friend could perhaps tell them that they should contact their current colleagues, and not a friend, to obtain a reference for their technical skills.