Well, that was fun...
A friend tried to change his wintertyres (very overdue) and managed to loosen this threaded end from his brake disc instead of the nut from the end... How is a mystery to me. He came over to see if I could help, but no tricks did the job and I didn’t have a welding station available. He will take it to a local workshop on Monday. Good luck to them!
Photo is a screenshot from a short video I took to send to my brother for a good laugh.