This will vary from one camera maker to the next, and perhaps from different camera models within the same brand.¹
Built-in popup flashes are, for the most part, spring loaded. There is always tension on the spring when the flash is stowed in its closed position. There's usually a small 'hook' of some kind that holds the flash down when it is not in use.
When the flash button is pushed, or the camera decides it needs to use the flash, a small servo motor pulls the hook back and the flash is released. The spring tension that is always there pushes the flash up. There is commonly a sensor/position switch of some sort somewhere that confirms to the camera when the flash is fully up and in position to be used.
Holding the flash down should not damage the spring-loaded mechanism that raises the flash. What it might damage is the little motor that moves the hook or the hook itself. But if your friend has been doing this for a long time with no ill effect, it's probably not going to break anytime soon.
I don't have a Nikon D7000 but I tried it with my Canon EOS 7D. With the 7D, if the flash is held down the servo motor moves the hook three times in about one second. When the flash does not confirm that it is up, the camera displays an error message in the rear LCD screen that says, "Er 05. The built-in flash could not be raised. Turn the camera off and on again."
If left in whatever automatic mode that requested the flash, the camera will refuse to take a photo as long as the flash is held down. Each time the shutter button is pressed the flash attempts to raise again and the camera throws the "Er 05" message and won't take a picture. Interestingly, just removing the obstruction from the popup flash and trying again without power cycling the camera works and the photo is taken. When the obstruction is removed and the shutter button is pressed again, the flash pops up and the camera takes the picture.
Obviously, your friend's Nikon is a bit different because it will take a photo while your friend is holding the flash down with their thumb. Fortunately, the D7000's flash apparently does not fire the flash if it is held down.
For other cameras, the flash may or may not be firing inside its housing while being held down! Some cameras will fire the flash while locked down. If this is the case the user needs to exercise caution!
Flash units produce heat as well as light. If the flash is firing inside the housing, doing it repeatedly can have serious consequences. I have read horror stories (and seen pictures of the damage) where the flash on a camera was stuck down/held down repeatedly and the heat from the flash was enough to eventually melt/deform the flash and/or its housing.
If the flash fires as it it held down while being used in an auto mode, it's highly likely it will be a full power dump. This is because reflection from the scene of light from the pre-flash fired in "Auto" flash modes is measured by the camera's meter. The difference between an ambient light reading and the reading from the pre-flash is used to adjust flash power. If the flash is being held down there won't be any effect on the scene from the pre-flash and so the camera will attempt to use all available flash power!
In the worst case scenario, such a condition might start a fire! You don't see or hear of it near as much any more, probably due to increased automatic circuit protection and temperature sensors now built-in to more powerful flashes, but back in the old days external flash units would sometimes catch fire!
¹ The question does specify a Nikon D7000. But the way stack exchange works, we DON'T need (or want) a similar separate question for every different camera model. Others who find this question should be able to see a more general answer that can be applied to other cameras as well.