Here's the reasoning - abs are a muscle group just like bis, tris, shoulders, etc. Do you train those bodyparts every day? Nope. A muscle group needs at least 2-3 days to recover.
Here's another thing - when you are trying to get the 6-pack, the muscles may be in great shape, but its FAT that is hiding them. Because we all know that you can't "spot reduce", you can't work one body area & expect the fat around that area to reduce. THAT is ALL about dieting.
And finally - if you are training your abs a lot, you end up building up your abs, i.e. you can end up w/ a thick, wide abdominal area, i.e "thick waist". That totally defeats the purpose of "the 6-pack look".
Seems pretty straight up to me. She'd be better off taking the time she spends doing abs every day & throwing in some cardio, preparing some clean meals for the next day or maybe some pilates to work core strength.