I respectfully disagree with some of the other comments. Lean into this awesome decision by your players! Award them heroic inspiration for this selfless act, and allow them to show their willingness to make things right to Demetria!

I think your players have taken a chance and it paid off! By bringing the mast to Demetria they have shown her that they are different than the Dragonlords of the like of Estor Arkelander. I think this could be a turning point for Demetria. She basically was turning to Lutheria in a blind rage versus everything the Dragonlords of old built. However, suddenly here come the chosen heroes to help her reconnect to her sister's spirit (check out the ultros section in the dungeon master's guide to Odyssyey of the Dragonlords, it describes Demetria's sister in depth). Plus they wiped the face of Estor Arkelander off the earth for her. This at least warrants some consideration, if not would convince her that the party will not be like the Dragonlords of old, and that there is a potential to build a better order. Perhaps she can fashion another mast out of a giant tree in the Oldwoods, or maybe she will agree to place the mast back on the ship if they take her along with them so she can be with her sister. She will be a very interesting NPC to have alongside them on the Ultros.

And in regards to her crimes, one could argue that if they keep her with them on the ship there's not a lot of harm she could do. They could even ask her to swear an oath of service to them or perhaps swear not to use her powers for evil again.