Don’t believe quotation websites

If the book became strongly associated with your friend, then a consumer protection exemption under "Passing Off" rules might allow the work to be protected but the onus would be on your friend to prove it.

Much like how, when one is in doubt in math, one just calls it Euler's theorem.

Textual evidence. The first demonstrable text wins, until an earlier is found. Sometimes it's fuzzy, because quotes morph (they get pithier over time). But most of the time it's clear enough, and the techniques for studying it are well understood.

Edit: The same is true in reverse, too: if a famous person really said X, it will be easy to find a precise textual source for X by googling. I don't mean a website claiming they said it (those are legion), but the original text, e.g. from their collected works, or an original newspaper interview. Since they're famous, that territory will have been trodden many times and there will be websites linking to it. Therefore if you can't find it, the famous person probably didn't say it. “Absence of evidence is evidence of absence” (hey, I wonder who said that one?) really works in this case.

I think that's funny as a question, but has the wrong premise. There is no 'actual' meaning of words; they have meaning only by mass human consensus. (What is the actual meaning of 'flumbiliable'?) By contrast, there is an 'actual' first utterer of a quote, whether or not there is any consensus on his or her identity.

(I suppose that you could argue, though, that it is only mass observation of a quote that makes it a quote per se, rather than just something I said; if I mutter something under my breath, and it later occurs as a widely quoted line in a movie, then should it be attributed to me, to the screenwriter, or to the character who said it?)