I'd say your buddy is right. The Longbow defines the die you are using for your combat check as your dexterity or ranged die. This means anything that boosts dexterity (such as skill feats or the belt) boosts this check. The fact that you are adding your unmodified strength die doesn't change this. You're just adding a die.
No one told him he couldn't use his dexterity skill feats with the longbow, did they? (Assuming he has any.)
Hi, my buddy who plays Lem has an understanding that when he uses a longbow in a combat check he can use also belt of dexterity because the use of a dex based weapon turns the combat check in to a dexterity check. He bases this on the following rule: Most monsters and some barriers call for a combat check. Weapons and many other cards that can be used during combat generally tell you what skill to use when you attempt a combat check (http://paizo.com/paizo/faq/v5748nruor1gk#v5748eaic9ram).
Rest of our group in the other hand has an understanding that a weapon cant change a check it only uses dexterity dies to resolve a combat check. So he cant use hes belts of dexterity with hes longbow in a combat check.
Your buddy is absolutly right. By using the longbow, the combat check gets some additional traits (as listet in the upper left of the longbow card) and since the bow determines the die rolled as his dexterity die, it also becomes a dexterity check. This allows him to use his belt of dex (and other stuff like snakeskin tunic) to add to his check.
EDIT: Ninja'd