Unfortunately I can't really see them becoming friends, even as adults. As teenagers they definitely wouldn't get along - Kate is too "not like other girls" to like Isabelle, and Isabelle has WAY too much going on to try and make friends with Kate - and as adults I just don't think the way they navigate the world is compatible. Kate would probably see Isabelle as airheaded, emotional, and weak, and I think Isabelle would pick up on that and wouldn't especially care for Kate either. I could see them having a few moments of connection, especially over Cy's eventual passing, but Kate probably wouldn't care to interact with another one of Cy's little girlfriends even if Isabelle was legal at the time. I'm very sorry, I wish I could give a happier answer T-T Thanks for asking all the same!