Bad customer service = piss me off

There are really very few things in life that will really get me upset. The seeing red kind of upset.
One of which is when someone treats someone else bad for no reason. Just such a thing happened today.

A little bit of background first:
A computer's warranty only covers the hardware of the system. Software is a whole other monster. Hard drive fails under warranty, you will get a new one installed, no charge have a nice day. But if you actually want your operating system and files (if any could be recovered) our boss makes us charge you for that. Sucks but we have to pay our bills too.

On to the story!
So a few days ago a guy comes in, laptop hard drive is on it's way to hard drive heaven. While he was here, one of our awesome techs who is no longer here (darn budget cuts) managed to save his most important files. Hurray!
He arrives today to pick up his laptop. As I was the only one here able to cashier, I had to ring him out. I let him know the price and get a "huh? I was told everything would be covered". Sigh
So I give him the usual speech run down of why it's costing him, blah blah blah. But then it dawns on me. Whom ever worked on it should have gotten approval to do the reinstall. Every once in a while the customer will have the know how to do it themselves.
I look up the call ticket and sigh. Of course. It's "HD connect to power supply" diptard tech that worked on it.
I go talk to him, inquire if he got approval first. He just stammers around. I roll my eyes and leave, attempting to think of a way to explain the situation to the customer.
Oh fluffy crackers, diptard is following me.
He insists on talking to the guy and proceeds to very poorly explain why the guy needs to fork over the money. His tone and word choices only serve to upset the guy.
Again he explains "but I was told everything would be covered."
Diptard: "okay. fine. I go erase everything then."
*blink blink*
No fluffy way did you just say that to a customer. Yes, that would be one way to solve the situation but for the love of tulips, don't tell a customer to his face you are going to delete all of his data!!
Until that moment the guy had been very calm and was just simply wanting to solve the miscommunication. Now, he was pissed.
Diptard continues to "explain" things but only serves to more piss off the guy. Hmmm yea I think telling someone "no no, you listen to me." is really going to make them not get upset.
A number of times I advised diptard to go get our manager. He would shake his head and refuse.
Oooo bad move there.
For a few brief moments he walks away to go check his notes. In that time I managed to calm the guy down and was just approaching a solution to the issue when DT had to come back and loudly exclaim that in fact the customer was a lier and had in fact approved the reinstall.
Round 2! Now the guy is really pissed. Explains to DT that he does not appreciate being spoken to the way he is (I couldn't agree more) and is getting rather upset.
At this point one of the older techs jumps in and lays it out that only the manager can solve this problem. Guy isn't too pleased but agrees to come back tomorrow as magically manager is no where to be found.
After he leaves, DT attempts to come up and "explain" his side to me. Not going to happen. I read the notes, no where did you state you got approval to do reinstall. Plus, you just treated a customer in probably the worst manner I have ever seen.
"You want to stop talking and walk away. Now" was my only response to him when he opened his mouth. I knew if he pushed it, some rather nasty words were going to escape my mouth. Amazingly he didn't seem to comprend what that meant and continued to speak. I repeated myself and same older tech stood between us and advised him to walk away as well. The words seemed to finally make their way through his thick skull and he left.