Good points so far. I'd also look at whether or not your friend was wearing properly-fitting gloves. Cold extremities and frostbite are affected not only by ambient temps but also perfusion. Pack straps might be an issue, but the major artery (subclavian) that might be affected is hidden underneath the clavicle to protect it from trauma; it should bear the weight of your pack fairly well (depending on the pack weight). tight-fitting gloves or elastic cuffs on your clothing can also influence bloodflow to your hands, particularly if they're swelling from hanging down all day.