you are correct in the beginning of your comment, but you've got the last part wrong. The visa is valid 6 month from the date it was issued (not 6 month from when he entered Thailand). The dates are clearly stated on the visa, in the OP's case it is valid until March 29.
You should complete your applications at a later date. Check the processing time for the embassy that you will apply in (should be on their web page) and add a week or two to the estimated time.
As others have said, check with your immigration office... but if you stayed at an hotel in Chiang Mai, I'm pretty sure you need to make a new one, since the hotel would have done one when you stayed there and that voids your previous one, meaning that you are now registered as being in Chiang Mai in the Immigration system.
Just a tiny comment for your information (and to not confuse others)... You don't get a "60 days visa on arrival" - you enter without any visa (visa exemption) and get a stamp that allows you to stay for 60 days. Visa on arrival is a different thing...
there is another worry about overstays... If they end up in any interaction with the police during those 3-4 days, i.e. random police check or immigration raid, they risk getting arrested, detained at IDC, prosecuted and banned to enter Thailand for 5 years plus they will have to pay for the deportation flight (chosen by immigration).
I'm not sure what document you are referring to as most embassies have their own information...
For example, The Royal Thai Embassy in Stockholm don't have any checklists on their webpage (just a description on how to apply for visas through e-visa).
I never did it myself from Bangkok... However I have done border runs from other locations in Thailand and I recommend flying to Malaysia and go back on the next flight (unless you want to stay...).