Head repairs on one side only

TL DR: Not an engine killer, but not the way I'd do it.

Not necessarily. There is an old adage when it comes to working on vehicles: If it ain't broke, don't fix it. The meaning here is, if you try to fix something which doesn't have a problem, you can introduce new problems, which once fixed can give you other issues.

That said, there are some considerations. The main one being, when you machine a head, you remove material from the head surface, which shrinks the combustion chamber size, which in turn raises the compression ratio of the cylinders involved. Raising the compression ratio raises the power level. As a rule of thumb, a single point of compression is worth about 3% power increase on a typical V8 engine (I'm assuming this is what is in the van, since it has two heads). This, in and of itself, would get me to do the same to both heads to ensure the engine stays equal on both cylinder banks. While this wouldn't be an engine killer, it will make it so things are not balanced, which might cause issues down the road.

I'm not sure why they went directly to machining the head. Most of the time a head gasket change would most likely fix the issue. They are the ones there who checked the head, though, so maybe they have a better handle on things looking directly at the head. Like I said though, if it was me, I'd do both sides at the same time to ensure power balance in the engine stays relative.

obviously I didn't see the vehicle but an external oil leak i would assume wouldn't be a head gasket at all but a valve cover... also i would almost certainly change the other side gasket if you got it apart already rather than rip it apart again in 20,000 miles why not?