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Worth bearing in mind that you're meant to declare dog aggression when insuring your dog, so while formal complaints about Barry will almost certainly achieve nothing, they'll definitely make life more complicated if he's insured. Plus if you had to claim against him for vet's bills it would leave him uninsured, etc - might be worth pointing this out.
But would also be inclined just to give him good kick.
Anecdotally, I've heard of him being obsessively humpy with a friend's female lurcher, to the extent that she now avoids them in the park when she sees them. But, I've certainly never seen his wildly monomaniacal aggression towards any other dogs, and the owner did say that this had never happened before etc.
But yeah, I think I agree with you - and if my sense of the owner is correct what might be best is to try to make it embarrassing for him - so letting Barry pop off whilst leashed around other dogs/owners etc. It is pretty wild to see and I think a few instances of that should hopefully incentivise owner to seek other parks (of which there are plenty around us).