I actually do think it was a little weird that Kaido didn't want her to go in and save anyone, considering he seems to have a fair amount of faith in her ability. (I mean she did save his life and everything.) After spending so much time traveling with her and hyping her up when she talks about her old party, I feel like him saying that if she couldn't save them all it would be better to just not go was ooc. Kinda seemed like the author just wanted her to have a "I'm a healer so I'll have confidence in what I can do" moment or something.
If he had just warned her that she might not be able to save everyone and that she might feel responsible if she can't, that would have been more in character imo. Not telling her she can't.