The stoma size will enlarge a bit when it is working, retract when it is calm - all normal. If it 'outgrows' her normal flange size, it's definitely time to check with her surgeon or GI doc. At 6 months post op my stoma needed to be dilated. Not a big deal, but worried me at the time.
If she is still concerned, if there is any soreness or bleeding, if it swells and doesn't return to normal size fairly quickly, if there is a noticeable change in color, then by all means get it checked for prolapse or some other issue.
Good Luck ... let us know ...