you didn't have a 90 day visa. You had 60 days and extended it so the stamp tells you what day you had to leave by. Ramifications of overstaying? Example: You get stopped for some reason, status is checked and you're on overstay. Then, worse case scenario, could be taken to airport and made to buy ticket and leave, fined, banned. You might get away with it but it's obvious it would be an overstay you could have avoided. Or. . they might just tell you to pay overstay on exit. 🤷
if he's not already done the extension before his 60 days were up, he'll be on a massive overstay already. No one knows what he's done because he hasn't clarified anything properly.
Did you extend this visa by 30 days already, so have the 90 days? But when you flew in you didn't notice you'd booked your flight for 96 later? Can you change your flight?