As posted in another thread, but applies here too.
We got away with this last year.
Someone dropped out, so another guy took the ticket, called Speed ferries to advise a change to the car details, add himself as a named passenger, but left the lead name the same.
On the day of travel, he arrived at Dover and said the person who was the lead name was sick and couldn't travel. "On you go sir, sorry about your friend"
No problems