Matthew Rick has said on his blog (visible on his author page here on Goodreads) that, "It's very easy for trolls on social media (besides me) to create fake tweets or claim they have "confirmed information," but I have released no spoilers about what happens to any of the characters, and I won't release any." Although it is possible for members of the Seven (other than Percy and Annabeth [see Kane crossovers]) to die, there weren't major protagonist deaths since The Titan's Curse back in '07. Nico is likely safe because of the potential outcry from killing the only gay character, and Reyna probably is the least safe, her upcoming POV notwithstanding.
Lexi Reyna. I think it will be a noble death that will help the rest of the 7 come together and overcome whatever their going through at the time. But that's just my opinion. Not sure, Uncle Rick always surprises us!!