I personally, would let it go. I had a co-worker who fudged everything, would buy stuff at Walmart like DVD player for trip, then return it and complain (fake) to get her money back afterwards. It used to make my skin crawl - but I stayed out of it.
Could you tell her in future you don't want to hear about this stuff? That's what I would do. Just say "You know, that's pretty dishonest - and it's our school lunch system. I'm not really ok with it, so I'd rather not know you do that."
If people shamed her by reacting to it, I think that would be more helpful if you want her to change.
Otherwise - if you feel strongly, you could talk to the principal. Be prepared for it to get back to her. I have no idea if they actually check this stuff.
ETA: B makes a good point. I hadn't thought of it that way, because other the person I knew who did it - did it to large chains and I'm sure it affects pricing (they must have to charge that back to the consumer somehow..) - I haven't known anyone who has lied to schools or child related programs to get a break.
Do they not have to show proof of some kind? We had a daycare that would charge less if you made under a certain amount - and we've had sports funding available for the same - you had to bring in documentation of your income.