The way to fix it would be for it to save in your character data how far you were when the first person in your party died. So let's you got the boss to 50% and then a party member died. That's your progression, because it's possible you were responsible for their death or didn't save them.

Then we should be able to restrict people who join to people who have been to 50% cleanly.

We should also be able to require that people have defeated the striking dummy for the final savage fight, because if they can't beat that they probably don't even have the crafted gear item level or other things like food and pots. Or they don't know their rotation or didn't do job quests to unlock basic job abilities.

That would solve a lot of the issues with Party Finder. But it's too complicated of a suggestion to communicate to developers who speak Japanese.