How should I cite presentation slides?
There are two practical purposes to scholarly citations:


acknowledging scientific contribution of others, the borrowing of ideas (mainly) and content (sometimes, in the form of quotes)
helping people find relevant content if they want to read it
bookkeeping, for scientometric/bibliometric purposes


In your particular case, you cannot fulfill #2 and #3, because your friend's slides are not available for others to read, and even if they were, random documents on the internet are not really used for bibliometric purposes.

So, you want a solution that will achieve #1, i.e. make sure his contributions are recognized by people who will read your slides. To do so, you don't need to give your citation any specific format. I suggest you simply write, at the bottom of slides you borrowed from him:


  Slide courtesy of John Doe


or 


  Slide modified from John Doe, with permission


In addition, you can thank him in your acknowledgements at the end of your talk.