my friend told me that in the very first lot of "double grass" you can get a second starter.
If you chose snivy as your starter you get tepig
If you chose tepig as your starter you get oshawatt
If you chose oshawatt as your starter you get snivy
My question is what would be the likelihood to find it?
This is untrue, starter Pokemon can not be found in the wild on any game.