Cereal is not listed as restricted item at USPS Mailing Rules for France I'd ask PO clerk ahead of time just to be sure.

@:-) Why don't you write a letter to manufacturer General Mills and ask them about availability of Cheerios in France?
If they like your letter, they might just offer to send some to your friend, at no cost

@:-) Perhaps your friend can have a chat with local supermarket manager - if that store sells at least some GM products, they should be able to special order Cheerios.

@:-) Do you know any FT'ers planning MRs from US to/via Paris? Find someone willing to:
- pick up a few boxes of Cheerios from their local store
- get in touch with your friend in advance to make these arrangements
- meet with your friend either at airport or in Paris

As a thanks for doing this favor, your friend can offer to:
- buy these FT'ers lunch or a drink
- give them a ride from airport to city if necessary
- play a tour guide while in Paris
- trade cereal for some french trinket, a box of gourmet specialty chocolates, a bottle of wine unavailable in US, etc

In US, Cheerios regularly goes on sale for $1.00 or $1.50 per box. Minus manufacturer's coupon $0.50 or $0.75, which many supermarket will redeem for double the face value at - $1 or -$1.50.
Making the total cost to the helpful FT'er(s) as much as $0.00.
If you need assistance finding the cps - just ask.