You're upset about them not refunding you a YEAR later? Unless it has a warranty of course they aren't going to refund it. Typically stores have a return period of 7-30 days, and after that you've had plenty of time to test it out. And like other posters have mentioned, it looks like your mare laid down in it in a puddle or something.
Did you test the waterproofing when you bought it? If so, and it worked, you got what you paid for. If so, and it didn't, you should have sent it back.
Or, did you buy it and then just put it away till this year, and realized it wasn't waterproof?
Either way I can not see the blame falling on the company. Waterproofing doesn't last forever, and the blanket wasn't exactly from an extremely high quality shop. Just re-proof it, it should work fine.