Putting bleach into a gas tank?

Hello friends.

My close friend will fill her ex’s gas tank with bleach. Even though I think this is a bad idea, I can’t persuade her otherwise. This would effectively destroy the car by killing the engine and gas tank.

She believes she can get away with it, but I have a good feeling her ex will find out. What could happen if he knew it was her but had no hard proof? And what would happen if she was discovered?