Also, a 1-participant plan with assets under $250,000 isn't required to file. Going back that far, it used to be a lower threshold for filing, but I don't recall off-hand when it changed.
If they were required to file a 5500 for 5500-SF and did not, the filing fee for the delinquent filer program for 27 filings is the same as for 2 filings. If the forms were prepared each year and just not filed, I would be inclined to file all of the late ones under the program, just to be safe. Even if they were not prepared at the time, the fees to get the forms done if you have good records should be much less than the potential late filing penalties.