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This is a tough one @Nick . I’ve had a few cases in the past with similar issues happening.

You’ve got some good techniques going with them already, like having the older dog muzzled for safety and advising your clients that his favorite treat should only occur when he’s around the puppy.

One counter-conditioning technique I’ve used in the past is “puppy gets treat while older dog sees – older dog gets treat one half-second behind puppy.” So essentially, puppy is fed (or fussed over or pet) and then the older dog is given a treat, praise, or pet (whatever’s most motivating to that dog). The other criteria of course is having appropriate distance between the two so the older dog is relatively calm.

What I want the dog to understand, quite simply, is that “when this dog gets something good, it predicts me getting something good!” My criteria at this point is pretty low – the older dog merely needs to see the younger one get the goodie and then is immediately given the reward. At this point I’m not as concerned with what the older dog is doing in terms of reaction; I simply want to start building the association that the puppy makes good things appear.