A leader is only as good as the people she surrounds herself with. That said, you still won't like being crossed by someone you mentored and supported in their career. Indeed, this person’s continual disagreement is starting to feel like a betrayal. But take a couple of steps back and ask yourself why you're so bothered by this. Didn't you say you wanted to only work with people who would tell it like it is? The difference between an ally and a yes-man is their willingness to say no to you. Hear this person out. They may be saving you from an impending fiasco.