The casino has been reported to be a scam for a long time. I have seen so many scam accusation both in this board and the gambling board.
Look at the negative reviews on their Trustpilot >>
https://www.trustpilot.com/review/betflip.io?stars=1Had you done a small google search before using the casino, you wouldn't have fallen victim. Due diligence should be your number one on your online transactions rule book.
I guess this is something he missed, 5.5 Eth is a big amount to lose just like that, and to lose it in an unknown casino with no announcement here is tough luck I've checked their domain registrar and saw this
Domain:betflip.io
Registrar:NameCheap, Inc.
Registered On:2019-05-02
Expires On:2022-05-02
Updated On:2021-04-02
Better report it on Namecheap with all the proofs you've got but before doing that follow the right format and post all your proofs here, screenshots, transaction records, it will help your cause.