your friend is basically right, the impact from a BB could set off a primer.
HOWEVER, without something to contain the cartridge its not effective. IF you can tape a cartridge to the muzzle of a BB gun, you can tape a cartridge in a close fitting piece of pipe (which will act a lot more like a gun barrel), and you can get some degree of directed energy from the round going off.
Yes, its unsafe, yes, its stupid, but in an absolute desperation situation, Assuming you had the BBgun, cartridge, pipe and tape, why not?
I'd have to be really, really desperate....
and odds are, if I have the cartridge, I'll also have a gun to fire it!
All else being equal (and it almost never is) bigger bullets tend to work better.