I was thinking of this bag here:
http://www.goldmine-elec-products.com/prodinfo.asp?number=GP23
the larger black components detect if something is inside its slit. You could set it up, so the ring drops into that component.
alternately this here might work if placed in a smart way:
http://www.sparkfun.com/search/results?term=ir+sensor&what=products
(i actually really love these sensors - they are super versatile much more so than one might expect.)
Can you get her a custom ring? one with a magnet in it?