Will the wife be held accountable for exempting her husband from the deferred dower after knowing that zakat is due on it, and is this considered an evasion of zakat?

The zakat (definition) on debt is paid from the wealth of the creditor, not the debtor. If the creditor does not have the money, the zakat is postponed until the debt is collected. Waiving the deferred dowry for your husband is an act of charity for which you will be rewarded, and it is not an escape from zakat. It is sufficient that you inform him of this.