You're absolutely right that there is a possibility her stone could get stuck at any time all by itself.
Since she knows she has a gallstone, she may want to seriously look into taking Gold Coin Grass or Chanca Piedra for at least a month or so first. These herbs are known as 'stone breakers' and have helped countless people. Also, drinking apple juice daily (if tolerated) or taking malic acid will help soften the stone(s) making it easier to flush out without getting 'stuck.' Gold Coin Grass and a lot of good information concerning liver health and function can be found at Julia Chang's Sensible Health site
There are simply hundreds of testimonials at CureZone alone by people who've successfully removed stones of similar or even larger size. I've just recently passed a a few calcified stones, one which was close to the size you mention - and didn't have a bit of pain from it. =)