And when does a song become co-written?

One of my own songs was definitely improved by a friend suggesting a change to one line. But if it ever became famous, I wouldn't credit them as co-writer, nor split the royalties with them. (Well, I might buy them a pint, I suppose!)

On the other side of the coin, I was once at an event where a well-known singer songwriter co-wrote a song with the group during a songwriting workshop. My sole contribution was the addition of the word 'and', which made one of the lines scan much better. I'm not expecting a royalty cheque any time soon!