Sportsbet.io should still privately to the client or even publicly state why the client failed the KYC procedure, while protecting the clients privacy, as they have done so previously:
I don't know if that really can prove or disprove multi-accounting. The client can be a jerk about it and say Sportsbet is lying even if the proof is correct. It's not something that can be verified independently (e.g. on the blockchain) so basically in order to use is as proof you have to trust Sportsbet anyway.