I agree you cant lie on your application since if they caught they will revoke your PR/Citizenship. There are two choices 1) Live with whatever he declared or Hire a lawyer to correct his mistake.Your post is super confusing. If one is married in a foreign country and doesn't declare that before landing, then it's a misrepresentation. There is a case of a male nurse who got deported because he lied during the process:
https://www.cp24.com/news/filipino-health-worker-set-to-be-deported-granted-temporary-resident-permit-1.5536679?cache=
Escario came to Canada in 2007 and worked as a live-in caregiver. He became a permanent resident three years later. But after officials found that he did not disclose that he had a wife and a child in the Philippines, they ordered his removal from Canada.
Escario admits that he made a mistake, saying he feared that his application would be delayed in the process.
Also, here: https://moyal.com/misrepresentation-upon-landing-can-lead-to-deportation/. Question 1.