The article does not state whether the mint was selling some of the gold coins to dealers at this time or not.
If they were all supposed to have been turned in, then the cashier basically stole the coins, no matter whether he sold them or gave them away. One cannot legally give or sell stolen property, even if the person receiving it did so in good faith.
However, if the government was allowing some of these coins to be sold to collectors at that time, then the government needs to return them to the family.
The article - which is quite sloppily written - does not state this.
But the historical fact is that these coins were never circulated, the coins in question were stolen by a mint employee, and were sold illegally.