Sorry to hear of your experience. My current car has diamond cut alloys and I dread the day I need to get new tyres fitted as diamond cut alloys do seem to be quite susceptible to damage by tyre fitters.
Hindsight’s a wonderful thing but with alloys (diamond cut or otherwise) it’s always a good idea to agree the condition your wheel(s) with the tyre fitting place before they start work and then inspect them again after they’ve fitted / refitted the tyre(s) before you leave the tyre fitters. If there’s any new damage, then it’ll have been caused by them and they should pay for repair or replacement of the damaged wheel(s).
Has the wheel ever been repaired before? If not, normally diamond cut alloys can be refurnished two times in a diamond cut finish, so it might be worth taking the wheel to another alloy wheel refurbishing company for a second opinion. Additionally, there are now some places that will do SMART repairs to diamond cut alloys, so that might be an option worth exploring.
I hope you get it sorted - and hopefully at the tyre fitting place’s expense!