I'm not suggesting you be an asshole about it, Linda. There are ways of letting someone go that are honest and humane, and it doesn't necessarily mean that you drop them from your life.
But what can I say, if mentoring or providing opportunities for your DP is more important to you than making the best film you possibly can than go for it. If making a difference is what you're after, I'd argue you'd be making a bigger difference to society by making a great film. And if your DP is more of a hindrance than a help in accomplishing that, I'd do what needs to be done. Again, it can be done compassionately.