even selecting it on his own
Instead to become mad trying to find possible solution(s) within the books, ask to the LH seat filler a reason about his/her behavior.
Ask him/her "why are you forcing me to do someting while I'm still flying within acft limits and company rules ? could you be so kindly to give me written reference about it, or it's simply your personal point of view ?"
It should be a good food for thought about CRM.