I'm sorry for your loss. I wonder how you ended up very unlucky.
But this does not automatically mean that the site is scamming you. Neither does it mean that the site is being dishonest when it claimed to be provably fair.
Try to go over your bet details and verify whether those rolls were indeed fair.
https://www.bitsler.com/en/provably-fair/verifier
https://dicesites.com/bitsler/verifier
His/her last roll must be the 13th. While it can happen that for 13 consecutive rolls you are getting everything red, I must admit this is highly improbable at 85% winning chance. I must say the player is extremely unlucky for having gone through such long consecutive losses with a very high chance of winning.