I think the way medical records are kept and shared depends on the jurisdiction and the legislation in place for the privacy of personal health information, which is at the provincial level. It might define circles of care in specific ways, so I don't think I could speak for all of Canada. What we said was that an assessor should be entitled to explore, with a requester, a range of collateral information. This might include talking to family members, if they have a good faith, bona fide reason to think it might be relevant and helpful, and also speaking with prior treatment teams.
A person can refuse, but the assessor should be able to ask. If they feel there's information they don't have access to that is pertinent to their assessment, they may be unable to reach an assessment of eligibility.