It's unacceptable because the refund of a full ticket does not require any calculation. It is simply antiquated process as most carriers can process a refund such as this on the spot and the customer will have a credit on their card within 2-3 business days.
At 25 days, I would have used the EC 261/2004 framework and turned it over to my bank. It puts the shoe on the other foot and you would likely have the credit long ago.