i think losing their job is a bit much if it's a first offense. This gate agent needs to be trained, not fired.
If training does not fix the issue, well... Then other options can be explored.
I am so freakin sick of "us" (flyertalkers) calling for people to be fired over minor crap like this.
training should be the first step. To suggest someone be fired, which impacts their whole livelihood, their family, etc over an upgrade? 
Yes, they should be called out, and yes management should be made aware. But unless it is a repeated issue that consultation and training fails to fix, to call for them to get fired is just petty and simplistic.