based on my own century riding experience, it's more about the tits time than speed so for a metric century, make sure that your friend is capable of spending at least 5 hours in the saddle riding. You can work on this by pushing the "recovery ride" mileage longer and longer before you step down for more than a stop sign or stop light. For a metric century I try and time my first get off the bike /Rest stop for 28-30 miles or two hours whichever comes first.

Admitedly my advice is probably one sided since I am, have been and will probably always be a solitary rider except on cross country stuff, so how to deal with a biking friend other than my sweet SAG guy, is a bit beyond my ken. I will mention that we decide, from the beginning, whether we are riding for speed and distance. If it is for speed, he will drop off at the halfway point, and if it is for distance, he ususally breaks off between 40 and 50 miles.

Good luck however you decide to ride.