He let his wife take care of the finances because he trusted her. He probably never considered the possibility that she'd take advantage of his trust. He was naive.

Something I've noticed about people who make above a certain amount of money is that unless they're super attentive, they tend to not notice missing money. Their jobs are usually demanding and they're busy people. When they do have free time the last thing they want to do is comb through their bank and credit card statements. Especially nowadays with automatic bill payment, if all your bills are getting paid it's especially easy for anything else to fly under the radar. At the height of her career my mom made really good money and I was a shithead teenager. I stole her debit card multiple times and she never noticed the money I withdrew from ATMs.