Soularstorm

OK - so here's the scenario .. you've written a song with someone else, and you're in the studio recording it. A guest singer comes in and adds a line that is quite memorable, that you end up repeating a number of times at the end of the song. Whilst it doesnt change anything fundamentally with the song (the song would still be the song without the line), it is quite hooky, so is that a scenario where writing credit should be attributed to the singer, and should you approach the other songwriter and tell them that you need to include the singer in the songwriting credits? Would love to hear from someone with a legal background, as well as people's interpretations of what should happen. What's the protocol? Thanks for any advice!