Why can't she do them normally - where is the weakness, her arms, her chest, her legs? Is she having trouble getting depth?
Try doing wheelbarrow walks. ie grab her legs and she walks around face down. Don't bend elbows much of course or she will fall down. Alternative is get on swiss ball, walk out from it and back again.
Try doing partials on one arm ... get into knees position at the top, lean to the right, go down a bit and use the right arm to push up, then alternate.
Try doing normal pushups but on inclined surface, e.g on a bannister railing or something, easier than normal pushup but still similar in form. ie at 45 degrees. Will help build up the strength for normal.
How the heck she can't do pushups is beyond me though.