I'm guessing your friend is heavy into "old wives tales" or simply got the application mixed up. Vinegar along with baking soda work pretty good together to clean off battery corrosion on terminals and cable ends, but "Boy Howdy"
don't put baking soda in the battery. The acid in vinegar is acetic and low percentage of volume ... the acid in a battery is sulfuric. Are they friendly to one another ... I don't know. I'd say someone told your friend that vinegar would bring a battery back to life or extend its natural lifetime ... don't hold your breath on that one.
