Quoted from delt31:

Only mentioning this b/c you seem eager to sell the machine. Current price likely has a lot to do with why it's likely not moving.....

Quoted from Yoko2una:

This is very common on BF, and they’re standups, not drops so they’re independent of one another. All three BFs I’ve seen (including one I owned until a month or two ago) suffered from this.
The solution is quite simple. The upper face standup wiggles loose due to the weird impact it gets. Underneath the PF, you’ll see the screw(s) that fasten it to the PF are loose. All you have to do is take that screw to your local hardware store and find a new wood screw that’s as long but one size wider. Or you can fill the hole and screw back in the original.
Either way this like the easiest of easy fixes. If your bro-in-law can’t handle this level of fix, Pinball may not be the best hobby to jump into, and if he does perhaps it shouldn’t be one as feature packed as BF as the more “stuff” you have under the glass, the more that can go wrong... but BF was certainly a fun game to own. Shots on top of shots!