Friend used a 12V power adapter instead of a 9V adapter without looking. Is my guitar effect pedal wrecked?
If the pedal is defective it won't be due to the adapter's power rating. Check if the voltages on both adapters are the same, and if their polarity is the same as well. Also check if it isn't AC where it should be DC.
For everyday's purposes 7W and 9W are pretty much the same. The pedal won't require 9W, so the 9W device will have some headroom, which the 7W device won't have, but it won't cause any trouble. If it would, it would be the adapter which would fail, not the pedal.  

edit
From the pictures you linked to in your comment (this and this one) it appears that they have reversed polarity and a different voltage. This could be bad: this kind of devices is usually not well (or not at all) protected against reverse voltages; they simply rely on you using the supplied mains adapter. Final.
How big the damage is depends on what's inside, but digital (like \$\mu C\$ or common logic) or analog (like opamps) ICs are probably gone. See if you can post a picture of the internals, so that we can assess how big the damage may be.