Yes, anyone can cast spells out of the Ring.
In-character, the ring was accompanied by a note(player's handout) that should give some clue as to Eugen's condition, and what the players could do.
It's been a while since I played this module, could any PC cast remove curse using the ring?
Could someone clue me in as to how we were supposed to learn that he was cursed in character? From what I recall we had no way of knowing.