I had a similar issue with the guy who used to do my hair. I think I started going to him around age 9, and he did my mom, and every now and then, my grandmother. I moved away and it wasn't convenient for me to go there (limited and no evening hours) but I still did. I dont like change. He was a lovely man, but a bit behind the times. He still did highlights with teh plastic cap, not foils.
I kind of realized my hair wasn't quite how i wanted it, and decided to find someone closer to home, which I did. I have to say I didn't handle it well, as I just stopped going (my mom was no longer in the area, so no awkwardness there). Sadly, I just found out he passed away from an accidental fall last Nov.
I'm not sure how you could phrase it, since you are friends, unless you're just honest with her, and maybe say you feel like you need a change, and while she's done a good job all these years, you're ready to try sometning new.