Why not just sit down and talk. The suggestion that you set a day aside to ride together is great. You need to ride your ride and shot for your goals. If you don't you will eventually grow to resent your friend and your friendship will dissolve.

My BF and I have had a similar talk. I certainly can't keep up with him so we will, from time to time, ride to the ride together, start out together and then he drops me. However, he is always waiting for me when I pull in and we leave together. Yesterday, I was ride captain for a 35 mile ride. He could have taken off with the fast pack but decided to hang back with me and keep me company as I swept the course. He was typically always just ahead of me but it was nice knowing he was there. He would be miserable if he had to ride at my pace all the time and I would be equally as frustrated if I tried to keep up with him. The important thing is that both of enjoy our time on our bikes, celebrate each others accomplishments. Talk to your friend and be honest (hard I know). If she is your friend she will understand.