How far out is the trip? While somewhat selfish, I would hold off on canceling her reservation until closer to departure. If there is a major schedule change or IROPs you will likely be able to get her ticket refunded.
For now, I would split her reservation out and monitor it for changes. If there is a schedule change have her call in and cancel. For schedule changes >90 minutes you should be able to get a full refund to original form of payment.
Yes, good call. You never know if things might change for the better and all 3 can end up making it. Wait until a week or so out when it is certain, then split the single person off the PNR and cancel it.