All of Beleriand doesn't need to know, but I think it would be weird if Boromir didn't know. And I didn't suggest that Boromir would base that decision only on her daughter's failed relationship, but rather on the broader problems among his people, such as envying the elves for their immortality. His daughter's broken heart could come in addition to that.
That story of Bereg I suggested, about Bereg being angry at the elves for not saving his grandpa (Bëor) and being "tricked" into leaving Beleriand by the bad guys lies, could make this problem more personal for Boromir. (And it could also give Bereg a backstory, not just him saying some bad things and leave, and it could be a way to bind the different generations together).
Maybe it could be a mix of both our ideas? Maybe Boromir doesn't know about Aegnor and only sees the immortality envying, but Finrod manages to see more and understand more when speaking to Andreth? Finrod speaks to Angrod/Aegnor and Boromir and they agree to give him Ladros? I don't know just doing some brainstorming