Jay wrote: My friend disagree's and say that only applies to weapons that have an Instant death special rule.
As Culven said, the ID
rule is when S >= 2T. Open the rulebook in front of your friend, and read the second sentence under Instant Death, "If a model suffers an unsaved wound from an attack that has a Strength value of double its toughness value or greater, it is killed outright."
Note the rule says "...an attack..." not from a weapon.
No weapon has the instant death rule, they don't have to, it is the strength of the attack that matters.
And this attack can come from either a weapon or from the Strength stat of a character.
The other exception that Culven noted is that some weapons or attacks might have a special rule that says they cause instant death regardless of the toughness of the target model. These weapons do indeed have a special instant death rule, but again, it is not required, it is an exception.