Okay, I just checked YS: 325, and the attacker's roll IS added to the damage of the grenade.
Rules look fairly simple from this point of view, add his attack to the weapon:4 to everyone in the zone, and don't allow him a defense.
[Edit] I know this probably won't do much harm past his armour, stress and such, but I'm not really comfortable with suggesting a consequence, especially when someone has paid points for a power designed to prevent damage. This way, he'll probably take some stress at least.