Re: ...in addition

No doubt it will do more damage.
Damage that could kill him. Which I highly doubt your GF wants on her record or conscience.
There are many strategic advantages to a tazer compared to a baseball bat.
1) is smaller and less likely to be handled away by the attacker
2) it may take several direct blows with a baseball bat to incapacitate an attacker, Tazers take a single pull of a trigger.
http://www.youtube.com/watch?v=y97Nj-QdDcM

3) Considering this is a young girl, I doubt she could put enough strength, or weight into the bat to cause significant damage.

They are not a 'single shot' weapon, they can be reloaded with a new cartridge in a matter of seconds for a follow up shot.
And even in the chance that you do miss, you can remove the air cartridge from the weapon and turn the weapon into a traditional tazer.(this can be done in a simple flick of the wrist). Basically, two weapons in one. Both non lethal. Tweet