Hi kg & welcome to the ladies' forum! COuple easy places to start --- even if she's very aerobically active:
1) DIET DIET DIET
2) She's 40+ => natural metabolic slowdown -> women tend to deposit fat in the middle so that's probably going to be where you see it the most if it does start to accumulate
3) No resistance training?
Sure you can buy something like YES to help, but you simply are not goiing to see miraculous changes w/o addressing at the very least a change in diet & training. I say both because the diet is probably the biggest culprit, but the age thing is a new aspect that she will have to fight even harder and if her body may have adapted to a lot of the aerobic activity she does, maybe a little resistance training in there to change things up & shock her body into havign to respond to a difference stimulus might be just the thing.
Its nice to think in terms of gettign a little bottle of soemthing to add to the mix - but its more changing the mix that will get the results --> and that means taking a look at her lifestyle --> diet & training.
I do know about the age thing - i"m looking at 41 next month --> but I've also successfully dieted down to 8% bf naturally for competition starting at age 35. I did experience a noticeable hormone change around age 37, but the fight w/ estrogen has always been there is probably the source of my 10 extra lbs my whole life.
I would say - get her to post up here -- post up the daily meal plan and lets start there. It would be great to see it listed here in specific detail, e.g.:
Meal 1: 8 am
1/2 c egg whites
5 oz chicken
1/2 c oatmeal
Meal 2: 11 am
1 oz almonds
3 oz chicken
etc
This can also go into a food counts program like
www.fitday.com to see exactly what she is eating - i.e. total calories & macronutrient breakdown (%, grams protein / %, grams fat / %, grams carbs). Then we have something to talk around specifically. I would suspect w/ her activity level that it would only require some small tweaks.
Also does she do any resistance training? One thing that people who do shitloads of aerobic stuff may enounter is muscle catabolism - if her diet doesn't include enough protein to support muscle maintenance and / or her fuel does't support the energy demands, the muscle mass willl suffer - thus you get the distance runner look - stringy, maybe skinny/fat.
These are my thoughts and I think this is the place to start before looking for supplements to make the difference. It just so happens that the old mid-section is where estrogen likes to deposit fat so that's what you are going to be fighting against.