AFAIK, Batteries in pokemon cartridges were only for the in-game clock, not the actual save files! So he is not safe!
Edit: i stand corrected! Ive never had or worked with pokemon carts specifically, but i have worked with & modded a lot of GB, GBA and N64 carts and not all of them have batteries, in fact most dont. I know n64 mempaks do, tho, but almost none of the carts. Info i had gathered from the various retro communities led me to believe that GB/GBA pokemon carts were a type that didnt need a battery for the save file, only the clock, but apparently that isnt always the case!