Must the husband refund the installments he received from the client who evaded paying the last installment for a program the husband developed, and for which he cut off the client's access to use the program?

Your husband is permitted to annul the contract and reclaim the software, or prevent the buyer from using it. He is obligated to return the money he received, and then he may dispose of the software as he wishes. This applies if the buyer delays payment. If your husband would be harmed by rescinding the sale, then he has the right to prevent the buyer from disposing of the software or benefiting from it until he receives the remaining agreed-upon price. This is analogous to the seller's right to withhold the sold item until the price is fully paid.