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He will declare the truth in pr renewals/citizenship/passport application. he will say the he is currently married which is the truth. nothing at any of these applications asks the marriage date, only marital status.
 
He could just marry the girl again in Canada. It is legal to marry the same person twice.
 
Why?, he do not need to. PR renewal and citizenship application does not require marriage certificate to be submitted and does not require the marriage date to be mentioned on the application. In addition his wife has PR already and do not need sponsorship from him. No need to re-marry here. What's your point?.
 
Your friend may be fine with leaving things alone. No one can really say for sure. He'll just have to live with the uncertainty and potential consequences.