To your first question, Yes he/she can stay upto 3-4 weeks without informing the IRCC, if you are you staying longer then that than Inform them through webform
To your second question, No need to panic about this situation. Officers are also well aware of the pandemic and also the restrictions because of that. There will no negative impact on that.
P.S:- my wife also visited me when our file was in process so I am saying that with experience. We didn’t feel any complications because of that. Stay Safe