Is it okay if I tell someone that with the mentality they currently have grad school isn't for them?
Why do you think it's not for them?
I think that currently they are not mentally fit for grad school.
It is okay to think that but you shouldn't tell people that. It is mean and people will think you are a bully for telling them they won't be able to achieve their goals.
I want to save them from all the stress before it happens so I think it's good.
I understand what you mean, but if the person really wants to go to grad school then saying it isn't for them is not appropriate. Telling them you don't think they have the right mentality for grad school isn't going to help them.