DFC, I don't think anybody is saying that a diversion is not acceptable, what the concern seems to be (certainly to me anyway) is that the student was sent off with the knowledge that there was fog at the first airport and that the superviion and briefing of the student seems to be a bit lax.
If the student did have to divert, it does happen ocassionally for unexpected reasons, then they should have phoned the instructor.
If there was a risk of fog, then the least a school could have done for a student is to thoroughly brief them on what they should do if the cannot get into any of the airports en-route. This could stop the student feeling hard done by when they had to do the whole flight from scratch, despite being authorised for the last one. When they plainly shouldn't have been.
I'm concerned that any school would send off a QXC when not only was fog forecast, but it was already there. Our students don't leave until the weather at ALL stops is above our minimums, not just FORECAST to be O.K. - First law of aviation, the Met man is always wrong!!
The instructor should have briefed all scenarios, such as diversion due to wx. In that way, there is no need for any 'free thinking' on the part of the student. They should know what they have to do in that situation.