Technically speaking, removing a node from an Availability Group (AG) does absolutely nothing to prevent the corruption of the SQL Engine or the Master database during an update.
The "SQL Engine" and the master database are local resources. When you run the Cumulative Update (CU) installer, it updates the binaries (sqlservr.exe) and runs upgrade scripts against the local copy of master on that specific hard drive.
Whether the node is logically "In" or "Out" of an Availability Group, the installer performs the exact same operations on the exact same files.
If the CU installer is going to "wreck" the engine, it will happen regardless of whether the AG is configured or not. Removing the AG configuration provides zero protection against binary or system database corruption.
The "corruption" your colleague likely experienced in the past happens during the "Script Upgrade Mode", the very first time SQL starts up after the patch. If this step is interrupted or fails, the master database can be left in an inconsistent state.
The Mitigation: The correct safety net is not destroying the cluster topology. It is taking a VM Snapshot (since you are on vSphere) or a File-System Backup of the System Databases (master, model, msdb) before applying the patch. If the engine breaks, you restore the snapshot/files. You do not need to dismantle your High Availability architecture.
Microsoft explicitly documents the procedure for patching AG nodes, and it relies on the Rolling Upgrade method where nodes remain members of the AG. They warn against removing nodes because of the re-seeding cost.
=> Reference: Upgrade and Update of Availability Group Servers with Minimal Downtime and Data Loss
The Standard Procedure:
Patch Secondary.
Failover.
Patch new Secondary (old Primary).
So please tell your colleague that removing the node causes a guaranteed headache (re-seeding terabytes of data) to solve a problem it doesn't actually fix (local engine corruption). The correct approach is:
Take a full backup of master and msdb (or a VM Snapshot) on PWGIRSQL2.
Apply the CU while the node is still in the AG (but Paused in Cluster Manager, as we discussed).
If it blows up, restore the Snapshot. If it works, you saved yourself 10 hours of work.
VP