Far too many assumptions.
While he was VFR at the time he may well have been IFR capable and ready and able to fly an IAP. Alternatively he might well have had another good and safe plan in mind. We just don't know.
On the other hand I would agree that it would have been polite to thank you for the information and indicate that he was IFR capable and able to "accept" the weather.
Whether or not its in the rule book I don't know but I guess if in doubt there was nothing to stop you pointing out that it was unlikely he would be able to continue VFR and that he would probably need to accept an IFR clearance in due course.
After that I guess its down to Darwin, there wasn't much more you could have done.
Thinking again. more subtly you could have asked him whether he wanted to you to try an arrange over the land line or via a hand over an IFR hand over assuming this was available to you. At the least it might have "shocked" him into relising that he was about to do something "illegal" if indeed that was the case.
Last edited by Fuji Abound; 11th Jun 2012 at 15:10.