Yep I would say a bit of both too, yes they almost certainly shouldn't have used a steamer but then did you tell them not to use one.... I know you didn't tell them not to use an elephant with sandpaper stuck to its ar$e either but you get my point...
As it is your property and you will be benefiting in the long run, i.e. if they had moved out and you decided to decorate the plaster might have fallen off even without a steamer, then I would say try a compromise and split the costs and lesson learnt.... hopefully if they are decent people they will see the logic:thumbs: