Does Ruined Rex damage happen before healing?

No, Ruined Rex's damage does not happen before healing. According to established RiftJudge FAQs (#10018, #10069, #10090, #10182) regarding the combat cleanup sequence, the healing step occurs before the resolution of the Deathknell ability.

Here is the step-by-step combat sequence:

1. Combat Damage Step: Combat damage is dealt. Ruined Rex and the opposing unit deal damage to each other.
2. Triggering Deathknell: As Ruined Rex is marked with lethal damage and dies, his Deathknell ability triggers and is added to the chain as a pending item.
3. Killing Units: Units with lethal damage (including Ruined Rex) are moved to the trash.
4. Healing: Surviving units on the battlefield are healed (Rule 461.1.a.1). Any damage marked on them is cleared.
5. Resolution: After the healing step is complete, the game returns to the FEPR (Finalize, Execute, Pass, Resolve) process, and the pending Deathknell trigger on the chain resolves, dealing 4 damage to the target.

Because healing occurs in Step 4 and the resolution of the Deathknell effect occurs in Step 5, any damage marked on a surviving unit during the Combat Damage Step is cleared before Ruined Rex's Deathknell ability deals its 4 damage.