If they want to ride with their mate and ride on the front, then they need to convince their mate to go up to their bunch, if it is deemed safe to do so.
+1
The issue isn't being in C28s and riding C28s pace, the issue is going in C28s and riding hard at the front because the pace is too easy for you. It's been an issue for years, since it's generally a pace a lot of people get comfortable with, and then they start getting strong but they feel safer or better staying in the same ride for whatever reason.
I know that if it's a small middies bunch and a large C28s bunch, it's not uncommon for C28s to start reeling middies in if a few people are pushing the pace. Even more so if it's windy.
If you want to train and C28s is too easy, get in middies and train. If you're strong enough to pull C28s half the way you won't be dropped.
If you want to cruise, go in C28s but cruise, don't get antsy because the pace is slower than what you can do.
Middies is not hard as training rides go, as long as you don't do too many long turns at the front. Sitting on is not much harder than C28s.
If people push the pace up for the 28s it makes it hard for the C25s to jump up as well. If you think going C28s to middies is hard think about how it is to go from C25s to trying to hang on to middies-pace C28s (ala C30s).