An acquaintance of mine has written, and performed at our local club, a very nice song.

But the beginning of it sounded rather familiar to me; and so, just for my own interest, I went searching on the internet.

And I discovered, that the first line of his song is almost identical to the first line of an already published song.

That already published song is not at all well known; I'm fairly sure that no one in our club will have heard it. And my acquaintance will almost certainly only ever sing the song locally, and never for profit.

If I point out to him the possible plagiarism, he might just stop performing his song altogether, depriving others of the chance to hear it, as well as being hurt and embarrassed himself.

So, what should I do?