If he pulled out in front of a car driving on the street and it struck his car, then he is At Fault. If he was rear-ended by the car following, then the following driver is at fault.
The driver suddenly applied the brakes.
Any sudden brake loss can be the cause of 1. brake line damage 2. master cylinder disfunction
Inertia
NO
Because you are decelerating sharply
Brakes.
You probably need to change your back brakes.
If all the passengers are properly buckled in and the car brakes suddenly to a stop, someone may spill their coffee.
passenger were hitched when the bus driver suddenly applied the brakes
law of inertia
You have a very serious and dangerous problem. More than likely the master cylinder if about to completely fail. When it does fail you will have no brakes at all. Have this repaired yesterday.
I am not a qualified expert on brakes but a brake job of that price seems fair if your brakes are completely shot and are in need of massive repair job.