My advice for him is to do quite a lot of research so he knows the questions to ask rather than relying completely on trusting a professional advisor. We bought our flat in Paris over 20 years ago so no point providing you with out of date contacts!
He should look carefully at “clause tontine” if they are not married or pac’sed. Pros and cons of using this clause so needs to reflect on their specific situation and aspirations for the future.
And he can write a will under English law, bypassing French succession rules but not inheritance tax (which is where the tontine can be useful).
Presumably if he’s been in Paris 10 years he is fluent in French, and knows the price of flats
My other piece of advice is to investigate thoroughly the coprop/syndic charges of any flat he considers. Don’t just believe what agent says but ask for documentation especially about works already voted.