if your mate serviced it on their premises but in his own time as a private "cash in hand" job then he shouldn't have used the dealer stamp. If its not gone through the books it's effectively not been serviced at that garage. This isn't considered a main dealer service though it's just like going to a back street garage.

If he's put it through as a staff/friends and family discounted job then it should have gone through the system and they should be able to trace it.

Anyway, how are they going to trace any of this back to your mate? How are they even going to prove you had anything to do with it. You may have owned the car at the time but who's to say the book hasn't been doctored since then.

Tell them it's not your problem. There's no need to involve your mate at all. If you mention his name he will be in the shit big time.