It's always a hard decision when it's time to move on in business relationships when you have a personal relationship with someone, but it can also be a positive conversation. Here is how I suggest that you may want to approach it:
"As you know, I have been struggling with growing my business and have been trying to take advantage of all of the resources I have. I am a member of an organization that works specifically with businesses like mine and there is a CPA who specializes in the PYOP business. I have talked with her and I think she can offer me some specialized advice that can really help my business grow and save me money on taxes. I hope you will understand how important of a decision this is for me, but I think she will be able to help me get to the next level."
......Insert hugs and warm fuzzy feelings here :)
I hope this helps!
