Not declaring a spouse/common-law partner is considered misrepresentation. It doesn't matter if he/she would give you any benefit. There is a lot of information and jurisprudence about it.
Benefit in this context is materiality: if it does not benefit the applicant, it is not a material misrepresentation.
If you are not sure about it seek a piece of qualified legal advice. While I respect @armoured for being active and trying to help as much as he/she can, in this particular matter he/she is mistaken to the extent that it is not misrep if someone is qualified on their own and not declared a spouse.
As you'll see below, qualified legal advice parallels what I wrote (terminology aside) quite precisely. If there's no benefit to the applicant, it is not material.
he did call an immigration lawyer today and the lawyer told him that this is not material. what it means is basically even visa officer thinks we were in common law during Pr application that’s not affect my case since there is no benefit for claiming single. The lawyer said if the common law partner is not Pr or Canadian citizen then this factor will become material.
sorry I am not quite sure about material too technical
No worries. In this sense "material" just means 'something that matters.' I phrased it in terms of benefit/advantage instead of materiality as it does get a bit technical. My example of height is a more obvious case of (non)materiality: since it literally does not matter at all, it's not a point "that matters" (material). Whether or not you were common law with a citizen is not material, since it would not have affected the decision - but as quite rightly noted, if the room-mate (potential common law spouse) was not already a PR or citizen, that could have been material.