+5. The +0 is the base of the strike, and it gets replaced by his base skill. Nice trick.

except the +0 is a +0 base as items and actions can modify it, so it does get replaced, and rules team agreed, so yay

melee sTrikes
Whenever one of your characters is going to perform a
melee strike, choose any other character within one rank of
your character to be the target of your melee strike. Roll a
d20 and add the strike’s ATK modifier to the result. If the
total is equal to or greater than your target’s AC, then you
hit the target, and inflict a wound (place one wound token
on the target).
The ATK modifier is the strike’s base ATK, plus all ATK
bonuses and penalties from items and other card effects.
Most strikes use the character’s Primary ATK as the base,
which is their first (or only) ATK modifier. If a card effect
sets a strike’s ATK to a certain value, then that modifier is
the base ATK (ex. “Perform a +3 melee strike”). Similarly,
when performing an attack (more on attacking below), use
the character’s second ATK modifier for the second strike
(and so on for any other strikes).

say what?

Not the first time the ruling has ignored what the cards say.