Technically, it's not dead, it's "undefinatly postponned, until the dev health get better".
So, the game coding he make himself (if any), and all the asset he made for it are his (/her) intellectual property.
It's rare to get legal trouble for using other peoples asset in doujin world, but it can still happen, and you have to take into account the values and morals of whoever might try to finish the work.
Some won't care, other will want an official endorsement fromt he dev before doing anything...