No, they are never allowed to be sponsored under family class if the PR holder did not declare them on the initial application.
A shockingly large number of economic migrants with PR choose not to declare family members because they think it will take longer to get their PR< despite the warnings from CIC that say "if you do not declare dependents, you will never be permitted to sponsor them."
Your friend has only two options:
- The common-law spouse can migrate to Canada on their own economic merits
OR
- The PR sponsor can leave Canada, renounce PR, and apply again as a new PR applicant and hope it will be approved.
Failure to declare dependents at initial PR application means a ban on ever sponsoring them as family class. Too many people choose to do it and then complain about being refused.