As frustrating as it is, try to let it go and keep growing. It's their loss in the long run — not yours.
Cheater

If you feel another member is using disallowed resources you should report them.
https://support.chess.com/en/articles/8562517-how-do-i-report-someone
That said, discussions of cheating, potential cheating, or cheat detection are not allowed in the general forums.
I lost with a cheater. No doubt. What can I do?