Think the prob here might be that it wasnt really his mate that done it.
Bottom line is that even with a mate doing it you should agree a ball parkish figure before he starts. If he's a mate and is "doing you a favour" because he's a mate, then he shouldnt be charging you the going rate IMO.
I wouldnt have let this other guy tell me I couldnt be there either. If he's doing a homer, then I doubt he's covered for anything if he screws it up and his employers didnt know about it.
Going rates should only be done when everything is going through the books. A mechanic aint gonna see £100 an hour in his hand if thats what Toyota charge you. And at least you're covered with them as youve entered a contract.
If the mechanic sees half of that, then thats what you should pay him, plus a few of quid for his time. Hardly a "favour" if youre paying normal wack without the contract.