In reply to bleddynmawr:
Is he doing the extended Inca trail/ world challenge type thing?
I took a 3 season bag and was absolutely fine, got up to 4000m+ I think (i'm no expert at this type of stuff). Incidentally the bag is still going strong 13 years later!
The extra bulk will be really annoying for the rest of his trip,if it takes in a lot of the hotter areas of the country too?
They wont go out in harsh weather so it wont get colder than zero or so, which will be fine in a 3 bag and some clothes.