battery swollen because of replacement charger?

A person bought replacement charger for his Macbook (I don't know the model) and he says that after using it a while the battery has become swollen. Now I think many would use the basic formula that chargers other than Apple made are bad but has anyone experienced similar thing where replacement charger is the one to blame? There are numerous cases where original charger has been used but battery has became swollen anyways.

Official explanation by Apple is that swelling is protection mechanism (which sounds like BS) and usually I think battery is the one to blame (as original chargers have been used and chargers still work after this).

I'm interesred to know your experience (not just opinions like expensive = good, cheap = bad) as I use a charger bought from same seller as this which is now blamed to damage batteries. I don't know how old his Macbook and battery is, my battery has over thousand cycles and still works great, even with this cheap charger. 2007 model.

Edit: and excuse my english, swollen, swelling, dunno how I should write that

It would nice to know the specs of his battery, I told him how to check them but it could be that he is not that interested in tech stuff and he will simply think that since problems appeared relatively shortly after new charger it must be it. Then again previous charger apparently had failed because he needed new one, was it original or replacement, who knows.