Just tell them you decided to go with a different provider. If you sincerely appreciate their help over the years, you might say something to that effect.
If they're your friend, they'll thank you for your business and wish you well. If they're not your friend, they'll question your decision or try to pressure you into doing something other than what you already know you want.
It's funny that we never see posts from financial advisors asking, "some of my clients are friends, and sometimes they leave, and I'd like to maintain the friendship after they leave, not for any business purpose, just want to keep the friendship... what's the best way to continue the friendship and allow them to feel comfortable with that?"
Those of you who are financial advisors, have you ever seen articles in trade publications to help with that question?
I didn't think so!