Did she lose all that weight as part of Basic? I know that routine is brutal and the food aint' nuttin to write home about. If she has the means, make sure she's getting enough protein, carbs & fat to support some muscle recovery. Is she having problems w/ the push-ups just from being burnt out from Basic training? (God love her for going thru that..) Is she having any pain at all from doing them, or just lack of strength? If its lack of strength, I would say, as buffchic said - start out with modified push-ups to start building back her strength. She can do the knee thing, she can also do wall push-ups and also if you have access to a gym, doing push ups on the bar on the Smith machine - its less of an angle than down on the ground and gives a chance to work up to the real thing. She can also do reverse push-ups I guess you'd call them -- lay under the bar on the Smith machine & do pull-ups instead of push-ups. And just keep eating good food to help her recover her strength.
Another thought I just had, if she's taken a beating going thru the program and losing lots of weight, is she taking any glutamine to help recovery? L-glutamine is fairly cheap and is a great recovery supp.