20+ years ago I had a few "lazy and weak" students. Firm and bold honesty solved the problems. USMC's statement about needing the ability to communicate to other crewmembers makes a good leader and Captain. Do yourself and the student a favor and have the talk. If they don't like it, they can shop for a new CFI. I dam sure don't want an F/O who is afraid to speak up if I'm screwing up. Do the profession a favor and own up to it, it is your job after all.
Cheers.