It's impossible, I repeat, impossible to get a covid extension if you were on a non-o (volunteer, ed visa, etc), since January 2022 (for almost 3 months). The price with agent went from 3.5k baht mid-2021, to 7k baht late 2021, to IMPOSSIBLE in 2022. When I say impossible I mean even if you throw 50k baht at them they CANT do it.
It should have been a red flag when they told him that they could give him a covid extension from a volunteer visa, its impossible. p.s: Ive been using the same agent for 10 years, never had an issue.
Your story is lacking crucial information: What is the agency name (FB)? What visa were you trying to extend? What mode of communication did you have with them? Do they have a physical office in Thailand? etc.
Khon Kaen is considered a "red zone". Took 45 min to get to Bangkok, but 7 hours bus back to Khon Kaen, lol! p.s: Train also require PCR or vaccination back to KK.