Deserter punishment comes with this drawback (among others):
Im not going to do the work of putting here the 1000s of posts about that debuff for you. You can look it up in the forums yourself. You want that?
Here is your issue.
You invited someone. You EXPECTED to finish the dungeon. But that guy maybe did not want to finish it. He wanted to time it. So he left, and has all the right to do so.
Next time. Be clear. Put “stay in key” or something in the title of the group. And ask in /party that everyone stays till the end or leave. Before you start.
DONE. Leaver problem fixed for you. All it takes is to talk to people, in a MULTIPLAYER game. Its not rocket science.