Water over the bridge now, but a good reason to book individual itineraries and one ways even if it is a big PITA to do so. That said, here is a suggestion that might get this fixed.
Call customer service and explain the situation and that you wish to have one of the six passengers split into its own PRN that consists of just the one way back. Tell them the reason that you want to do this. You are wanting to cancel just the outbound portion of that one PRN so it will not make a mess of the reservation for six people. You should end up with a single booking for the five remaining and a OW for the person with the missing outbound. Do not be surprised if the easiest way to do this is to cancel and rebook the entire itinerary as six individual ones so that the missing outbound can be canceled. You may end up with that passenger having unused TTF's for the cancelled leg of the trip on their name, depending on how it is handled.
This can be done. Whether it will be done is the question.
You may have more success contacting our personal FT gurus,
southwestwhit, southwestgabe, or the others by PM with your dilemma. This is the sort of thing at which they excel.