what time is your wife doing the cardio (upon waking up/morning/afternoon?) Ideally she could do empty stomach cardio in the morning in which she wouldn't eat anything prior to doing her cardio then come home and have a post workout meal.
post workout could be something like whey protein powder in water and oats.
If she works out int he afternoon it would be harder to do cardio in a fasted state (as it would be in the morning) but if she threw some weight training possibly in the mix and doing cardio afterwards would work well too