Since the license is from outside the EU, EU law does not specify exactly what the rules are so there might be differences from one country to the other within the EU. Being an EU citizen does not make much difference either. Worse, for US licenses, agreements are on a state-by-state basis, some licenses can be exchanged, other cannot. Fortunately, Ohio is among the states that have an agreement with France regarding driver's licenses (here is a permanent link to find this list).
All the rules are explained on service-public.fr. It does not sound easy… Your friend will in any case need:
- Form “cerfa n°14879*01” (for some reason it must be printed in color), filled and signed
- Color copies of the foreign license (both sides)
- Official translation of the foreign license
- Two copies of a proof of address (i.e. utility bill)
- Two photographs
- Two copies of his passport
- Proof that he has been a resident in France during the past six months (rental contract or tax return; an “attestation d'hébergement” is not enough)
- A certificate from the consulate of the country where he obtained the license that he was a resident there at the time he obtained it.
Depending on the situation, the préfecture might demand other documents, the page explicitly advises people to enquire beforehand. According to the same page, it's only possible to exchange a license in the first 18 months after moving to France. After that, you apparently need to resit the test in France.