I didn't want to say anything before, but I will now.
Let me preface this by saying I've never had any dealings with the shop in question, so there is no bias.
From the way the OP was made, and the slim details that were posted, my first instinct on this was user error. "Only a couple weeks" after an engine swap is a long enough time to **** up an engine. If it started knocking on the way home, that's one thing.... but a couple weeks later sends a little red flag up. Put someone behind the wheel of a new more powerful engine, and they'll likely be in the mood to give it a good workout.
Now that more details are out, this is clearly the fault of the owner. First of all, if I got an engine swap and needed an oil change done, I'd be going back to the show that did the swap so they could check out the engine anyways!! Make sure everything looks good. Why take it to another shop? They obviously ****ed something up huge. Unless of course the owner was actually doing it himself and ****ed it up due to lack of knowledge or butterfingers, who knows. Then he's even more to blame.
Frankly this entire thread is disgusting. It's amazing how some people will just jump on the hate train without any knowledge of the situation, just because they have had some sort of past issue with a shop. It's pretty immature and pathetic. If anything, this should never have come up. If the owner of the car wanted to voice their opinion, they should have posted on here (or joined up to voice their opinion, if they aren't currently a member). It's fine being an intermediary, but it comes with the risk of not having all the facts.
An expensive lesson, for sure. But one I'm sure the car's owner won't forget.