Steely Dan, your player is correct, magic weapons impart keywords to the powers in which they are used with IF that power has the Weapon Keyword. Let's take a look at a Frost Weapon, normally it is only a +x weapon, when the At-Will ability is triggered it imparts the Cold keyword to any power and ALL damage becomes cold damage from the power. When the Daily power is used it also imparts the Cold keyword to the weapon, but it adds xd8 of cold damage (All other damage remains the original damage type(s)) and the target is slowed until the end of your next turn. (Now according to the example on PHB 226 the weapon would always have Cold as keyword whether or not the at-will is used or not, but I think the example is wrong based upon RAW)
PHB 226:
PHB 226:
Like racial powers and class powers, magic item powers often have keywords that indicate their damage or effect types. When you use a magic item as part of a racial power or a class power, the keywords of the item's power and the other power all apply.