Sounds entirely reasonable to me. At the school where I work we've had students who were originally from Poland or Lithuania, so it's been arranged for them to take a GCSE in their own language without it being taught at the school. Also, at the end of DH playtime we assumed that fifth and seventh year students would take their exams at the Ministry rather than the damaged school, so I see no reason why a French student could choose to take an English NEWT if he so chose. I rather imagine he'd have needed to apparate to the British Ministry to take it with a suitable examiner, though.