Gameboy keeps turning off when battery pack is tapped lightly

Hello! I’m borrowing a gameboy from a friend and its a Gameboy Advance sp. I am uncertain how long this has been occurring, or how long the device has been owned by my friend, but every time the back of the gameboy is even lightly tapped like tapping my thigh during a bus ride or setting it down on my night stand,to simply touching it with my fingers as I play mother 3, it will turn off the gameboy and I don’t know how to fix this. I asked my pal and he said it’s been like that since before I borrowed it. I think either its the battery on it’s last limb, but he also has an extra battery on the back so im not sure if its removable or not. please help I havent owned a gameboy since 2nd grade and i’m a bored highschooler who needs entertainment.