So a little update. I called the hotel, and a subsequent follow up to SPG confirmed this, and apparently it looks like I'm SOL.

What happened was that my dad ended up paying for the hotel as it was to be his treat, and according to SPG only he can claim the points/stay credit because it was his name on the credit card. Unfortunately there's nothing they can do about this, as it's in the T&C apparently. Oh well!