First and foremost, I wouldn't discuss discipline with a dissenting husband in front of anyone else. The bystander will probably have opinions, if it's a child they'll likely hear criticism of themselves, and since I'm trying to promote GD over whatever DH's method is, it's going to come off as criticism of him. I only rarely meet a person who can take criticism, and rarer still is the one that won't simply stonewall the conversation around an outsider.
Secondly, just like with the kids: start with modeling. He'll see the results. If he doesn't, he's already got his back up against it and there's no way forward without a fight.
Third, don't do to him what you wouldn't do to the kids. Don't nitpick, reprimand or scold, at best he'll resent it and treat you like his second class mother. Even if he's doing something less than ideal RIGHT NOW, you're going to make more headway by being supportive. Promote the good and shunt the bad out of the limelight.
I have no credentials, but my husband believed spanking was a necessary evil and I had to intervene to prevent it at one time, but he has come over to my way of thinking.
Pagan Quaker maman of two.