Thank you everyone for your suggestions. The subject is quite complex!

On further investigation, it seems that the part of the song I thought might have been copied was itself likely copied, from an older song which is out of copyright.

I'm pretty sure that the person who sang the song would have copied whichever song it was unintentionally; so I agree, it can't then be described as plagiarism.

I've decided not to say anything at the moment, especially as he's an acquaintance rather than a friend. He writes and sings quite a number of songs, so it's not like this will become his 'magnum opus' and be continually performed.