Radiologist here. There are multiple safety zones (four to be precise) around the machine and extensive screening procedures are required to access the inner zones. The magnet of an MRI is always on and extremely strong. However, you need to be pretty close for it to pull a gun from your hands. Like, less than a few meters. That would be zone 4. He should never have been that close.
The button he pressed is called a quench. It’s for life threatening emergencies only. Think “patient trapped between the machine and a metal object.” It vents the liquid helium used to keep it superconductive and basically destroys the machine, but the magnetic field dissipates in minutes. There is a way to wind the machine down without destroying it in situations that aren’t life threatening or for servicing, but it takes hours for the magnetic field to dissipate and even longer to bring it back.
This was almost certainly an older style of MRI that didn’t use superconductors. You could turn these off and on, but the strength of the magnetic field was much lower that what can be achieved with superconductors.
I also looked up the wind down time and I was mistaken. It’s a day long process to wind down and wind back up and MRI and do all of the testing and adjustments, but the loss of the magnetic field happens in a few hours. I edited my post.