You are correct that it's odd. There is no glue in there. The glass rhinestone is held down with thin double sided tape and is kept from any sideways movement by means of tubing that is pushed over the button shaft.
I have never heard of one coming off while using normally. All the force is downward and should not come off by itself. What might have happened is they put a VC in without its cap and got the rhinestone stuck in the cap or something along that line. Or was just curiosity. Something I'd do...
Regardless, the solution is to re-install the stone. Clean the surfaces and use some type of thin double-sided tape. Carpet tape will do. Stick to stone, then trim around it. Use tweezers to put back in place. Advise him not to use without as will melt the top of the button. If the stone is lost, I can send you a couple new ones.
In the case that a VC was used and the top of the button is melted, all is not lost. A small drop of JB Quickweld will suffice as a heatshield. Just a small dap to cover the top of the switch being careful not to get anywhere else.
To deal with the tubing, you need to use a paperclip and bend one end the fashion a small hook. This jig can be used to snag and remove/position it.
Please PM me with any questions.