If they were never married, but filed under 'common law', as defined by the state of residence in 2014, then it's possible they must file married filing separate until a divorce is obtained. Again, this is completely based on the state laws for 'common law' marriages.
The statute of limitations may or may not be reached depending on what was on the tax return that could potentially still be linked to the current business, Schedule C. On the other hand if you filed as married when you were not legally allowed to then, as @ColeenD3 indicated an amendment must be filed.
For the current year, you should file the return correctly and the business portion would carry on like it was always filed by the person who owns the business, assuming that only one of these taxpayers owned the business. Enter the information for Schedule C using the prior year(s) returns.
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