Your damage is based on not just strength, but also weapon damage, weapon ability and a teeny tiny (tiny tiny) bit of dexterity along with any damage you may have on armour/jewellery.
For example if i had 300 strength but a dagger that only deals 50 damage and my dagger ability is only 300, my friend with 100 strength, 500 dagger ability and a dagger with 80 damage will have a more statistical damage (damage value on stats page). Does that make sense at all?

well he did mention same gear, so i assumed its Daggers ability (and idk about dex giving dmg so havent noticed)
also can be that ur friend is using poison weapon, with good amount of dex and maybe rings
every 25 main wep ability pts (if u use a dagger, then the main ability will be daggers), give 1 dmg
im not 100% sure on the number, but it should be around that amount