I dont know if your hub was working a recovery program and this was a slip, or if he was a dry drunk, on a binge...whatever the case, why the disappointment?
He went right back to AA you say...focus on that achievement. Alcoholics slip and relapse,,part of the disease. Its not failure, its sickness.
If his cancer returned, would people be disappointed in him?
Get yourself and kids educated about the disease, buys books, get to 12 step meetings..al-anon for you, alateen for the kids.
Dad isnt a failure, he is a man with many illness that wish to kill him.