If they have what looks like a permanent break-up, they are supposed to let CIC know. The spouse will be excluded from the application and does not get PR. The principal applicant gets PR unless they were relying on spouse points, then they may lose out too.
A visa that has already been issued can still be invalidated. However, if they have already landed, they will both remain PR.
If they split up during the visa processing and only the principal applicant gets PR, should they get back together, there is a way for a PR to sponsor a spouse. However, it can take a long time and they would have to re-qualify as a couple, if common law, live together for 12 months or if they want to avoid that, they could get married. A sponsored spouse would have to cohabit with their sponsor for at least 2 years after getting PR. Otherwise they could lose it again.