What is her diet like now? If you can make small tweaks over time she may respond better than if you just hand her a brand new eating plan.
Basically, just like you, she ought to be getting about a gram of protein per lb of lean body mass.
She needs to eat plenty of fruits and veggies.
She ought to limit her carb sources to foods like oatmeal, sweet potatoes, yogurt… the less processed the better. That doesn’t mean she can’t have a bagel or sandwich occasionally, she just shouldn’t make it a main food group. Simple carbs (white bread, rice, cookies, high sugar foods) cause an insulin spike that makes the body store excess calories as fat. If you avoid the insulin spike, you’ll avoid extra fat storage.
She needs to eat more frequent smaller meals (5-6) and drink plenty of water every day.
Healthy fats are important. She can get those from nuts, nut butters, salmon, fish oil, or avocado to name a few.
Here is why she needs to eat frequently. The metabolism runs two different ways, anabolically and catabolically. When there is a steady source of nutrients the metabolism stays in an anabolic (muscle building/muscle retention) state. When her blood sugar dips too low and for too long (too many hours between meals) the metabolism switches to a catabolic state and begins breaking down muscle stores of glycogen in order to give the blood sugar a boost. You don’t want that, especially not if you are dieting. You want to preserve as much muscle mass as you can. The easiest solution is to eat more frequently.
Healthy fats (saturated, polyunsaturated and monounsaturated) will actually help her lose fat. Each gram of fat contains 9 calories, more than twice as much energy as 1 gram of carbs. On one hand that means she needs to be careful how much she consumes, on the other hand it means that she’ll get more energy out of a handful of nuts vs. a handful of crackers and that her body will expend more energy metabolising the fat. I.e. it uses more caloric energy to digest.
So basically:
Protein with every meal
Carbs with breakfast (body is best adapted to use carbs as fuel vs. storing when you first wake up)
Veggies and healthy fats with every other meal during the day
Carbs after working out are also ok… if you’ve had a taxing workout (not just a walk around the block)
To keep it simple she can shoot for a palm sized serving of protein for each meal, a fist sized serving of carbs with breakfast and one or two fist sized servings of veggies with her other meals. Eyeballing things will probably work better initially than counting calories.
She can try things like snacking on raw veggies if she feels bored or hungry as well as eating her meals off of a smaller plate so that she doesn’t feel deprived (small plate makes the meal look bigger).
That’s all I’ve got off the top of my head, feel free to post more questions if I forgot to cover something.
Oh yeah, the elliptical and walking are a great start but she could also benefit (and will see faster results) if she incorporates some weight lifting too. More muscle = faster metabolism = faster weight loss = healthier mom.