Did you discuss at the outset what your training would entail? Did you anticipate that the slower friend would not have the time to get overly serious about it? How often are you expected to ride together? If it's just once a week, can you treat that ride as a recovery ride? Is there anything positive you can do to help your friend get more on board?
I appreciate the importance of health and fitness goals, but I also understand the importance of friends. Rides come and go, but it's hard to find a good friend. If you can, figure out a way to not alienate your friend in the process of training for something you decided to do together.
I was fortunate that when I first got into cycling, a number of my friends exercised some patience as I got stronger. As time went by, I grew stronger than some of my friends. I remembered the kindness they had extended to me. Since then, I've struggled to train as much as I would have liked, so now the shoe's back on the other foot yet again. Fitness fortunes can change from year to year; I try to keep that in my mind when deciding how to train with friends.
Live with intention. Walk to the edge. Listen hard. Practice wellness. Play with abandon. Laugh. Choose with no regret. Continue to learn. Appreciate your friends. Do what you love. Live as if this is all there is.
--Mary Anne Radmacher