Oooh, Dandelion would be miserable in a way he would never ever admit. He would, in theory, move on but quietly always would yearn for Basil. Something that ended up making him feel complete was taken away from him, and he would forever feel less because of it.
I don't know how much he would miss Hell in general, but Basil would dearly miss the little surprisingly comfortable corner he made in it. He would quietly miss the adventures, the friends he made, the way he felt like he could be his true self, and, of course, he would deeply miss Dandelion.
Their relationship gets to a point where they are more miserable apart than what ever dire circumstances that comes with being together.