How much later after making the 1031 exchange did they sell the now principal residence?

They keep it for the required 2 years.  Do they recapture depreciation taken while a rental property?

In this situation they were supposed to hold it for 5 years after the 1031 exchange.

The deferred gain is taxable. 

If they hold it for 5 years is the deferred gain still taxable?

Deferred gains will eventually end up taxable, youre deferring the taxability, not excluding it.

If the replacement property now principal residence is held for 5 years after the exchange, it is eligible for the Section 121 exclusion, assuming all other criteria are met. Gain to the extent of prior depreciation will be taxable.