Well it depends on the atmosphere... What is the AC's attitude like? What is your attitude like? How well do you get along with the AC? Then it comes down to the problems with the AC and what, if any, steps you have taken to try and fix the problem(s). Have you talked with the AC and let him/her know the issues? Has there been ample opportunity for the issues to be addressed? If you talked with the AC and they refuse to change anything, then it's a matter of telling him/her that it's best for the team... If you are friends outside of coaching, it might be hard at first but be professional about it.
I am an assistant coach and I would want to know if I was causing more harm than good for my team... Again, just be professional about it no matter what the AC says or does - because if they decide to scream and yell at you, that gives you the perfect opportunity to show them through their own actions why you can't have them coaching on your team.
Were you the one with the SS who played Catcher and the mom and dad being unhappy about this? If so I feel for you as I feel the AC will not be the problem but rather the Mom. Good luck. There is not easy way to let a coach go. I would get right to the point and not get mad or let your voice be raised in any way. You must remain calm when he will not be. Remember, if you feel it is right then there is no need to worry. Just do it and move on. It will be painful for a moment but make the team better for a longer time.