How should the captain handle that one? When the captain asked for 30 nm and the copilot advised 15, the captain should have keyed the mic and made his request directly to ATC. Nothing prevents him from doing that.
When the F/O continued to be insubordinate, the captain had full grounds to take that approach to the chief pilot following the flight. Further, if it was one leg of several, at the next stop the F/O should have been put off the flight and the flight terminated until a suitable replacement could be found.
I would have no problem at all taking that kind of attitude to task...and yes, if that attitude is persistent in the cockpit, then the F/O should certainly be dismissed.
It's one thing to speak out about a safety issue. However, when the captain makes a conservative call and the F/O reduces the margin of safety, that's the exact opposite of being positively assertive. That's dangerous. The F/O doesn't have the right, nor is it his or her place, to advise the captain that he's being too conservative.
I wouldn't put up with it for a minute.