Yes of course they should, that is one of the things they sign up for when they take the job. The correct course of action for her would have been to resign, you subsequently still remain silent on detail, but are free to challenge those argueing the case for the course of action you originally objected to.

As someone pointed out above, Clare Short, with all the information she was privy to, did vote for action in the case of Iraq. She threatened to resign but didn't, only finally resigning once Saddam Hussein had been removed from power. She now comes out with the claims about the British Secret Services. Compare that to the actions of Robin Cook - he did it the right way, Clare Short did it the wrong way and she should face the consequences for doing so.