that's not correct. If he's on initial 90 days then can stay untill his last day. If he's on an extension then needs to leave bu midnight the day the wp is cancelled. You're confusing this with the extension denied stamp that costs 1900thb. Op will have to apply for an extension that is then denied. You don't get an automatic 7 days to leave.
it depends if he was on his initial 90 day non b visa or if he's on an extension. If he's on the initial 90 days then he can stay untill the last admitted day . If he's on extension then what you say comes in to force.
I last visited there in March 2020 (13th,,,, 2 days beforw lockdown) I wouldnt call it a dump,,, it was clean and all that..... more like a service station on the m6