They could have refused to comply with an unfair request granted through back channels via the french political police and a landowner's mafia.
Go to court as a host, let the prosecutor present "proofs" that this person deserves to be prosecuted under international law, which they don't have because this is a clear case of political repression. Let the judge laugh at the ridicule of the case, or worst case scenario comply with the judge's order, which would hopefully give enough time to the targeted person to know they're a target by now and stop using Protonmail altogether (or do it via tor).
That's what a responsible host does. I don't have the same standard for smaller, non-profit hosts. But for a big organization with vast sums of money like Protonmail i would expect no less.
What's next, cooperating with chinese political police?