I don't usually get involved in post such as this but what the heck. I don't see you being charged a single supplement, you booked for two and were charged for two, but unfortunately, one did not show. She paid for her fare by paying it to you, you've lost nothing and were charged nothing extra (supplement). That person loses their part of the fare. It's not as if you booked for one person.
As I see it you owe her nothing and as she apparently did not follow the insurance rules, they owe her nothing as well, nor does the cruise line. I don't see how anyone on the next cruise could help with the situation either.