"You should go home, You are not fit for life here"
As others have been saying in the comments, this statement is too direct and is likely to be taken as an insult. Saying that someone isn't "fit to live" somewhere is like saying they don't deserve to be there (or, worse yet, that they ought to be killed because they don't deserve to live there.) It's the kind of thing an evil villain might say in a movie when making a threat.
You need to soften the language, especially since you are talking to your boss in a professional context. Here is one alternative that would probably be more suitable for you:
"You should consider going home, this job doesn't seem to be a good match for you"
Here's one that would use the word fit:
"Perhaps you should go home, maybe you aren't the best fit for this job"