Specifically at 5th/6th level it deals (4 x Weapon) + modifiers, so if he's using a 2d6 weapon that'd be 28 average before modifiers so this would be a slightly above average one (though maul might have a thing where it re-rolls ones and increase the average to ~32 before Str, feats, and magic weapon, making this an average one). For comparison, an encounter power for a 23rd level barbarian deals the same base damage (that from Str, feats, and weapon will increase slightly) with a 1 round debuff.