Clarification on counterspelling please., Did I just burn my pretty dress? Options

[30+coyote]

So i'm GMing a 4th ed group and I had an odd thing happen. Normally our combats are not close quarters however in this last adventure two members where caught alone in a small apartment when they where attacked. The mage over-casted for like 12 boxes of stun. He however said he could cast it and both protect himself and his compatriot from it while in the area of effect by counter-spelling his own spell. Now could someone clarify for me, if you have a 12 hit stun-ball and you counter-spell it for you and your friend for 6 dice does that lower the effect of the over all spell or does it just lower the effect of the spell on him and his friend specifically. If you can overcast and then null the effects on just you then it makes mages even more awful powerful.