Sounds like he had a blowout from overdoing it for the first part of the ride. You caught him on the hills. Don't worry about him. It's not that big a deal. Drafting is a two-way partnership, if there is even acknowledgement of the partnership. The person in front needs to be sure the people behind are staying on, but the people in back have equal responsibility to tell the person in front if a gap opens.

Grog put it very, very well. And don't let appearances fool you into thinkig the person is fitter or more skilled than you. Observe and watch first.