It does sound like he needs something in his riding, any fool can ride along in a group badly, stretch the distance and the pace and he'll need to switch on. But you're not alone, this sort of thing happens in Mickey Mouse road races, anyone that's fit can have a pop.
To 2tyred. It's easier to be blunt with people you've ridden and raced with, those that have seen you at your best and also seen you when your a broken man. It's harder to comment on another mans riding when you don't know them, though on the road it pays NOT to be PC.
As for ride leaders. How on earth does everyone know where they're going every Sunday every week of the year. Most clubs have a rota. Like now I'm plotting a new route for Sunday. It's 68 miles and it looks like 18/19 mph average should be doable (for this time of year) I also need to route find on the go so I hope the group will take my lead