There is an error in description of the Slay skill in the rulebook. The language for parrying for other characters was accidentally left out. Consider the paragraph below to be part of the Slay skill.

It is also possible to parry an attack directed against someone else as long as they are within weapons length.  A character cannot use a ranged attack to parry an attack for another.  The character may block any low magic or Nature Magic spell that is not combined with a Perfect Aim skill for another character, but they then take the effect of the spell and must call an appropriate defense that will stop the spell in order to avoid the effects of the spell themselves.