Ok, first of all. IF this garage has charged Zirks friend / relation for a track rod end they never fitted that is outrageous. To then fail the same part again a year later and try and charge AGAIN they need stringing up.
However, there are a lot of assumptions here.
It's been for a test. It failed on the OS TRE. Zirks then realised (and the tester probably should have too, but that's not the issue) the same item was replaced last year. The garage are now Presumably doing the part for free under warranty. There's nothing wrong with this. New parts do fail. I see it everyday.
The problem is IF they never fitted the replacement part and / or are lying about its failure then and now.
There's no way of proving this without either having the old part in your hand and / or jacking the car up and seeing if it has play yourself.
So without this info what are you going to say to VOSA? That the garage replaced a part under warranty for me so they're clearly dodgy?
Obviously that's assuming they do replace the part under warranty which I'm sure Zirk will be able to see when he takes the car away.