Of course the info needs to be included in the paper if it cannot be found elsewhere. I didn't want to imply otherwise. Whether the friend needs to be a co-auth

Of course the info needs to be included in the paper if it cannot be found elsewhere. I didn't want to imply otherwise. Whether the friend needs to be a co-author is of course up to them and certainly depends on the size of the contribution, but that is beside the point of both citing the source of a certain idea and making sure the reader is able to verify its validity. In fact, I personally lean towards often including arguments even if they can be found in sources you cite. If you can provide a much clearer argument than the source (and it doesn't detract from your own work), you should include the improved one in your work. If it is a minor detail that is both worth citing as well as easy to include, then I think you should include it to avoid requiring the reader to hop around from paper to paper to get an understanding of your work. You should make your work as accessible to the reader as possible.