Is he using it to code or just chatting up his AI girlfriend.

Joking aside, if he is one of the top developers in the company and if he is "actually" a good developer, when compared to others outside of the company, then I can see this bill.

The current feature that I'm working on, required 100 messages to finalize things and I would say the context window was around 35k - 50k per "chat completion". My model of choice is Gemini 2.5 Flash which has an input cost of $0.30/1M. Compare this to Sonnet which is $3.00/1M.

If the person was properly designing and instructing the LLM to build something advanced correctly, I can see the bill being quite high. I personally don't think you need to use Sonnet 99% of the time, but if somebody else is willing to pay the bill, why not.