Look, how would you feel if your dad did something horrible under the influence? Shot someone, hit someone with his patrol car, shot HIMSELF? My guess is, even though it wouldn't be your fault, you would find it very hard to live with.
I'd go so far as to make an anonymous call to his Sergeant if it kept him and others safe. If they stopped him before he went on duty and ordered him to get help or else, it might be the push that he needs. And their hand wouldn't be forced because he would have been stopped in time. Administrative leave, with rehab, might save his pension.