Abs are made in the kitchen. The muscles are there but there's flab over them. My abs don't show up until I'm around 10% bodyfat. Good core work will help build abs - but particularly for women who are not likely to have a well-balanced training regimen, any ab-specific workout you start doing w/o also looking at the diet and tweaking it to get rid of the fat, is probably just going to be promoting getting thicker. You build ab muscles, like every other muscle, it will grow. For women that usually means thicker. Probably not what you were really going for. And to repeat as always, you can't really spot reduce, so 10MM sit ups is just going to get you back to overtraining that one muscle group and promoting thickness.
Training:
- squats
- DLs
- excellent core stuff: supermans, situp and hold for counts, trunk twists, etc. Here's some other stuff you can also google with "core stability training abs":
The Best Core Exercises and Workouts
At Home Workouts: Killer Abs and Core Stability - YouTube
Summer Ab Workout and Core Stabilization Training - YouTube
But IMO more importantly is what does it mean "the diet is in check"? If its not producing the results she wants, then it needs to be tweaked. And specifically you're looking at losing bodyfat. I'm a huge fan of carb cycling because its easy to do and doesn't leave you starving or having to make massive changes in how you eat generally.
I'd suggest by listing a current daily meal plan and even better if you can include portions, and use fitday.com or whatever food counts program to list the specific macronutrient breakdown (%, grams of carbs, fats, protein) and total cals. Also what are your wife's stats.