I'm not sure what the rules are in Canada but here in the U.S. the pilot has to have the field in sight, the runway in sight, or a succeeding aircraft that is also landing at the field in sight, *before* being issued a visual approach clearance. If there is a preceding aircraft for the approach and the pilot in the succeeding aircraft does not have it in sight, but has the field in sight, then radar separation needs to be provided until the #2 aircraft has the #1 aircraft in sight and can maintain visual separation. Again, this is in the U.S. Canadian rules may differ somewhat.
As to the other part of your question, I don't know all the radar sites involved, but the loss of one radar site does not necessarily mean that there is no radar coverage at all. It could be that there is still coverage, but it might be degraded or spotty. But if there is no coverage, I believe the aircraft can still be cleared for the visual approach, but radar service will be terminated once there is no radar coverage. In that case, the pilot will be responsible for separation from that point on until landing, unless there is a control tower operating. In that case, the tower would provide separation.
Hope this helps.