[pyre]

Not all games have a separation of user data. Some games basically only have one save slot, so that starting a new game wipes out the save data.

[freehunter]

This caused me to be beat upside the head with a Gameboy repeatedly when I was young when I hit "new game" on my cousin's Pokemon Red.

[HackR]

If you were my cousin who did that to me, you wouldn't exist today, to put it lightly.

[nopassrecover]

This was done to me by cousins and brothers maybe 4-5 times (with very well developed games before each time). I understand the frustration (although what you describe sounds a bit full on) particularly as I'm pretty sure you had to start a new game then save.