You sure about that? I don't think I've ever needed anything above +1 to damage any werecreature at all.
And he did say he switched to a silver weapon, which is still +1, and was able to hurt the creature then.
Helaman-did you by chance have magic weapon or greater magic weapon cast on your rapier? If you have that spell cast twice on a weapon it bugs the weapon and you have to re equip it for it to pierce DR.