If guy buddy won't slow down to your pace, don't ride with him on this important ride.
Last weekend i found a great compromise with my guy buddy. I rode the 55 mile loop and he rode the 80. He rode with me for 25 miles AT MY SPEED and then we kissed goodbye and did the rest of our rides at our own rates.
YOU MUST COMMUNICATE THIS with your bike buddy. HE can ride any speed he wants but if he wants to ride with you, he has to let YOU be in charge of the speed. Good luck.