Have they already failed the residency obligation? If not, then it is always possible for them to meet it.
Being a U.S. citizen gives you an easy way to enter Canada as a tourist. it doesn't do you any favors from a PR perspective. They need to enter Canada before the 3 year anniversary of when they become PR's. Or else, they may loose the status. They could always try to enter the U.S. posing as visitors, but make no mistake that after 3 years passes there is always the risk that they will be discovered, which may mean them loosing their status and needing to leave Canada.
Best way is to meet the RO. Let them come back on their 3 year anniversary by latest (by which time your citizenship application should be submitted.) Let them stay in Canada until citizenship is granted. (Processing time can be up to a year, but can also be quicker.) Then once they are citizens they can live wherever they want.