I see no problem with having an old tumble dryer and just keeping it for emergencies, it may be less efficient than a more modern one but you need to take into account all the energy consumed when manufacturing a new one and the raw materials consumed in making it. That may well make the older tumble dryer the more efficient.
Fires in tumble driers are very much a recent phenomena so for that reason as well on older tumble drier may be more safer as well. I always think the most energy efficient way to run any machinery, whether car or tumble drier, is to run them until they fail and only then replace them.
As for occasional use, that strikes me as admirably ecological. There will be times, like, for example this year's wet spring when constantly drying clothes on racks indoors is causing excessive moisture and condensation indoors and it is necessary to dry things in a tumble drier.
What I wouldn't have done was ask someone to stop the tumble drier and take things out. I would simply explain that it was much better for the environment and global warming if, where possible, clothes are air dried rather than tumble dried. He will have to live with global arming far longer than we will.