All that would be needed is for another passenger to complain/ask for a CRO and say that a service animal can't encroach on the leg room of other passengers. If it does, then the owner needs to purchase an extra seat. Airlines are not required to accommodate service animals that take up more than their assigned seat. I would've booted the owner, not the innocent couple. You can't have an oversized dog and not expect to pay for an extra seat.