Parent help?

My friend's Dad wants to buy a motorcycle, but her mom said no because it's dangerous and she doesn't want to waste mais money. But he said he might get a job transfer, and it would be a safer route, save $ on gas, and he found a really cheap motorcycle.
He told my friend that he'd thinking about just buying and not telling her mom.
Should my friend tell her mom that her dad might do this? She's really afraid he'll yell at her (he yells a lot). Or should she just not tell her mom, even though she knows?