Did he cook/smoke the sausage yet? If it's still uncooked, he can correct the amount. Just un-case the meat and mix in the required additional TQ and restuff.
Yep, what Rick (NEPAS) said is spot on. For mixing with ground meat, the correct amount according to Morton is 1 1/2 level teaspoons of TQ per pound of meat (1 1/2 level tsp is also the same as 1/2 of a level TABLESPOON when calculating for multiple pounds of total ground meat). If using as a rub or brine, the amounts are different.
Also if the recipe originally called for Cure #1 or "pink salt" and you are substituting Tender Quick, make sure to cut back on any additional salt you add as TQ has a significant salt base where the other cure does not. Otherwise you may have an overly salty finished product.