Can I use mortar to transition a height difference between cement board and drywall?

A family member helped me install some cement board in my shower, but the cement board and drywall aren’t level. It already has two coats of red guard and is basically ready to tile. Can I use thin set to level this out, let it dry, and then tile as normal on top of it?

Pictures attached. I am reallyyyy trying to prevent having to rip it all out.

Any solutions would be unbelievably appreciated. Gotta get this done by next Tuesday.

Thank you in advance!!