I think developers accidentally racking up unexpected thousands in costs on their first AWS project is a pretty common phenomenon that their support has standard rules for handling.
The developer said the agent deployed multiple CloudFormation templates, I'd bet that AWS waived the charges for the unused resources - like EC2 instances that were idle most of the time, very high margin SKUs, etc.
Now, for 100 Gbps of egress (which didn't actually happen) - and this is grounded speculation - I don't think that AWS would give a discount that is greater than CloudFront rates.
100 Gbps is A LOT of data.