There are several solutions on corporate level or mass-distribution but as he was using CloneZilla, a clean install before joining the domain would be the simplest option to make an image.
Yes it is possible to just remove and re-add the computer to the domain but anyone working in IT knows nothing ever goes the way you want it to and its never just that simple.
It's been that simple when I've had to do it .
Delete from AD > Re-Add to AD > On system, put on Workgroup (remove from domain) > Re-Add to Domain > Done.
I've had to do that several times when domain-join didn't work right in SCCM at where I work now (i.e. I gave the wrong credentials, or added to wrong group, etc.).
But yes, taking an image before adding it to the domain would be best.
However, you were suggesting that he performs a clean install on all the lab computers (unless I was misinterpreting your post).