Yes, in this case you would measure from any one of the three pins to the core of the stator
It may be more obvious however if it IS burned - note that most stators will appear dark chocolate brown, even potentially black, at the sections that receive the least oil cooling. That does not necessarily mean it is bad - but obvious visual burning probably already means it's bad but would be confirmed by the isolation check, either way.
Example - see below
I would expect that stator is absolutely fine.
There's really no good outcome here unfortunately - the tech may indeed have been right, but either way, the guy is still out the cost of repair.
I'm not at all insistent it was probably not failed - may well be - would just have liked to see confirmation before engaging in such an extensive/expensive outcome.
Maybe a little more 'feel good' (or at least better) for your neighbour if indeed it is proved to be bad.
But there would certainly be no satisfaction on my part in finding it was OK.