> Generally people put a part time job on the back burner and focus on the main full time job.

And if "people" was only the manager in question that would be somewhat okay, but usually it's the reporting chain above of that person that thinks the same too.

I made the mistake of accepting to manage an intern (not mentor, manage) once, and I was expected to do 100% of my normal job on top of managing him. It takes an unbelievable amount of time.