Brake booster replacement instructions please?

My son approached a stoplight and applied brakes to stop vehicle. The vehicle came to a complete stop. As soon as the vehicle stopped he heard a loud noise followed by a hissing sound and then brake pedal went all the way to the floor. At this time, the brakes were totally out, not functioning at all and he coasted into a parking lot. I don't have his keys at the moment. I popped his hood and checked brake fluid. Brake fluid reservoir level is fine and I see no sign of leaking fluid at brake booster, master cylinder or anywhere I can visually see. With vehicle not running, brake pedal goes straight to floor back and forth with absolutely no pressure at all.

Can you tell me what possibly went wrong and/or possibly diagnose this problem, please? Thank you!