Dependent Visa - When It Can Be Filed

I have a query regarding the dependent visa. If my wife has to get a dependent visa and has to appear for the interview, she will need a No Objection letter. Please correct me if I am wrong.

Also, if she resigns from her company, does she needs to wait for the notice period to get over so that she can obtain the resignation letter from her company to appear for the interview ? If not, is it possible that she can provide a mail from her manager as a proof of her resignation ?

For dependent visa, H4 or L2 , applying in india , applicant does NOT need NOC from current employer