As much as I dislike being a wet blanket, I doubt he's going to get anywhere with Kubota, unless he can bully them into replacing the boom. He's dealing with two separate entities here, one being Kubota, and the other his local dealer. It sounds like the dealer is on his side as far as the labour is concerned, but they're not willing to pay for a boom as well. Kubota has refused to provide them with the parts free of charge, but they are hoping to keep him as a customer if it doesn't cost them too much to do it.
Like it or not, it IS out of warranty. Kubota has no obligation whatsoever other than good will and good relations with a possible repeat customer.
The weld probably was defective, I don't dispute that. It's too bad he didn't notice the damage prior to the warranty expiring, if it was there at that time.
I've had excellent warranty service from my dealer on the one occasion I've needed it, beyond the expiry date I expect they MAY help me out if they can to keep a customer in the long term.
Was extended warranty available on the BH-76? I know it is on the tractor itself, the implements may be different.
Sean