Well, it would depend of course. But the instructor has, in the past, already signed out this student for a first solo, a XC solo, a QXC and possibly a few other "firsts". So a short positioning flight should not pose a problem.
The most important consideration, IMHO, would be the frame of mind the student is in, after having failed the skills test. That's why I would insist on a phonecall after the exam, regardless of the outcome.
But technically speaking, if the student is ready for his skills test, then a short positioning flight should not be a problem at all.
And you can also look at it the other way. If the student passes the skills test, then by law he is entitled to circumnavigate the world, without any further training at all. That's what the instructor supposedly prepared him for. So if the instructor is not confident the student can do a short positioning flight on his own, then it's the instructor who should be failed.