My mom perseverates on sweets, especially ice cream. Last Monday she ate 6 drumsticks in one 24 hour period. We took her ice cream away and parse it out daily from our home next door (after a reasonably healthy dinner), but she got her medical driver to stop at the store and she bought ice cream. She completely forgets that she ate that much in one day and wonders why we are letting time pass before her next ice cream. She makes upset baby faces when she is denied ice cream. This is not my mom!!!
There's also a disorder called pica, reflected by eating strange things, although I don't think ice cream could be considered in this category of nonfood items.
Have you duscussed this with her doc?