The ribs on trap barrels are not parallel to the bore. They are low at the muzzle and higher at the chamber end. That's why they have a built in vertical lead. When you lower the comb you will lose sight of the front of the rib, before you get the desired lowering of the POI. HMB
ClayC, yes, what you propose, putting on flat ribbed-barrels, will work just fine, now that you have assured us that your wife's stock can be dropped enough to use them. And the test for that - seeing only the receiver - she has carried out correctly as well.
And the flat-ribbed barrel, once she is sighting right down it, will probably shoot fat, which is what she is after. But it's worth checking anyway once the barrels are on, if only to be sure that flat-shooting is really what's happening.