Just to agree with a couple of other posters in here....
I am assuming you recieve the updated report once airborne.
That being the case, the formal planning stage of the flight has passed, so I would assume that you have dispatched and become airborne without an alternate (something you do in Oz but we dont do here in JAR land).
At that point there is no legal requirement for an alternate.
On reciept of the updated WX info, prudence/airmanship would dictate that any number of appropriate options be considered i.e. an enroute diversion, return to origin, land at destination below published minima etc etc...
I think the point I would have highlighted to the trainer would be..it depends on the SPECIFIC circumstance
