Your friend's concern about waiting for the microwave to stop beeping before opening the door is based on some truths, but it's important to clarify the details.
The magnetron is the component in a microwave oven that generates the microwaves used for cooking. When the cooking cycle is complete, the magnetron stops producing microwaves, but the microwaves that are already inside the oven continue to bounce around until they are absorbed by the food or other surfaces. This is why the microwaves do not instantly disappear when the timer reaches zero.
So, while the magnetron itself powers down when the cooking cycle ends, there are still microwaves inside the oven that will dissipate naturally over a short period.
The reason some microwave ovens continue beeping after the timer reaches zero is to alert the user that the cooking cycle is complete. It also serves as a reminder that the microwave is still hot inside, and opening the door immediately could result in steam or hot food splattering, causing burns or other hazards.
It is generally safe to open the microwave door once the cooking cycle has finished, and the microwave has stopped beeping. Waiting for a few seconds allows any residual microwaves to dissipate further, reducing the risk of exposure to radiation, but the microwaves inside the oven are no longer at dangerous levels after the cooking cycle has ended.
As a precautionary measure, if you are concerned about hot steam or food splattering when opening the microwave door, you can still wait a few additional seconds before opening it completely. Always use caution when handling hot containers or food to avoid burns.