I've heard stories where people pulled a card from a pack, sent it in, and it came back as evidence of trimming
Since I believe you are a stand up guy and to me you've proven to be one, you should do what you believe is right. Whether that is issue a partial or full refund, not issue any refund, or just straight up block the guy...it's up to you. I'm not sure there really is a straight up right or wrong answer. My gut says not to issue a refund, but that's just my opinion...just seems weird that it's been over a year and he just now gets around to it. If I were him, there would be no way I would request a refund. If he waited that long to send it in, it's on him and he should eat the card...I would expect the same if I were him. I'm sure he's frustrated, but this seems like common sense to me and he should probably just move on.
Good luck and keep us updated,
Mike