I bought a ticket for myself and a friend to both go to the Mayo Clinic in Minnesota and the end of October. His appointment will likely only take one day however I may be called to stay longer for additional follow-up tests.

We are both traveling on the same itinerary, using an AMEX companion certificate. So traveling to Minnesota won't be a problem however what happens to the certificate arrangements if I have to depart later? Does that screw up my friends travel or PNR? Is that allowable by Delta, and are they amenable to changes?