Well -- we generally say "abs are made in the kitchen". If she's training but wants to get the bodyfat in her waist reduced, that just means she is a typical female having to deal w/ estrogen-pattern fat depositing. Women have estrogen. Estrogen, by nature, promotes fat depositing in that area from thighs to midriff to protect a fetus - just basic survival design. So sometimes depending on how each person's body tends to store fat, its harder to get that tight look.
Since your body reflects (responds to) the lifestyle you lead, if you want to drive a specific change, you need to tweak some part or all of your lifestyle - meaning diet / training / cardio / recovery (sleep + stress mgmt). Specifically for bodyfat loss you can't do spot reduction - your body just won't respond like that. Since there's a higher concentration of fat cells in certain areas, e.g. waist & hips for women, work on tweaking the diet to get over the hump. I"d say review her diet to see if there are some things she can optimize - doens't mean she needs to go on a spartan competition diet, but to get to a specific goal, approach it like competition prep where there's a phase of specific diet/ trainign geared towards the goal. There may be just a few simple things she can do to optimize the diet. Also if she doesnt' do cardio, consider a couple 20 minute sessions of high intensity interval training (HIIT) - walk / sprint / walk /sprint / walk / sprint, 3-5 min cool down.
Exercise-wise - mostly I'd just say don't do shitloads of crunches w/ the intent of "working that area to cut down fat" - because you can't do spot reduction. Further a lot of things like weighted crunches, side bends, etc. are buildign your muscle, not specificallly cutting fat and can actually make some people look blockier in that mid section. BTW I'm NOT saying some ab work isn't good - just not piles of it thinking that the more you do the leaner your stomach will be. Its just another muscle group to work.
But approach the waist area as a bodyfat reduction effort. If she's already fairly lean and muscular but tends to hold bodyfat there, she will need to step up the diet and maybe cardio a bit or more generally intense training for fat burning becaues she's partially working against nature there (that's why its so freaking annoying for women....its how we are designed...)