Perhaps try posting on askgamblers, they might be able to mediate your case, but I am not sure if winna has a representative there. I would have suggested casino.guru, but as far as I know, they don't handle cases that have something to do with sports betting.
You are correct. Casino Guru does not handle sports betting cases and Winna is not listed on AskGamblers. So, it won’t be possible to file a complaint against them there, either.
If they are so sure about their decision then why did they offer him a refund of ~$900?