Travelling to Croatia from France with a French residency card

Here's my friend situation, he's a refugee with a 10 year residency in France (with a residency card valid for another 5 more years) and have a refugee travel document that expired last year. We're planning to go to Croatia for some holidays later this month (1 week long).

We know he doesn't need a visa and that French citizens don't need a passport to enter Croatia.

All third-country nationals who are holders of valid Schengen documents, as well as national visas and residence permits of Bulgaria, Cyprus, and Romania do not require an additional (Croatian) visa for Croatia.

Citizens of France may enter Croatia with personal ID.

http://www.mvep.hr/en/consular-information/visas/visa-requirements-overview/france,74.html#p

But we assume he still needs a valid travel document and because Croatia isn't in Schengen then the residency card isn't considered as one. He asked to renew his travel document but it is unlikely that he'll get it prior our holiday.

Are we right about our assumption that a new refugee travel document is mandatory ?

What would happened if we "weren't aware that he needed one" and tried to board the plane ? (either getting stopped by the airline personnel, the French passport control police or Croatian passport control police, on either flights (France->Croatia and Croatia->France))