Well,
The thing about an emergency descent is that you have to descent to 10.000ft ASAP, terrain permitting. Coming from a flight level you have to make corrections for nonstandard atmospheric conditions, hence the requirement for a local altimeter setting and a temperature.
Your friends answer for finding the nearest airport's QNH is correct, this should be done however before you fly over an area that you are about to cross, not during the possible emergency descent.
The question, what if there no airports because for example deserts or ocean.....
I doubt you will find any oceans where the obstacles will reach above mean sea level!!!! Not too many deserts above 10.000 ft too!!! There are a few mind you, but they do have airports where you can get the altimeter setting ( gobi desert for one).
Some countries publish regional altimeter settings though. So if you find yourself over a country with a large desert above 10.000 ft check the manuals you are provided to obtain the regional altimeter setting.
Personally, I think, that is a far stretched question!
Edit, I think, if your company regularly flies over areas with high terrain they will have to provide you with procedures to comply with regulations!!
Last edited by flyburg; 19th Jul 2013 at 16:11.