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There would presumably be a big wadge of repayable APD on that fare. You can leave OLCI until 70 minutes to departure, but not much more than that, particularly if there is an API issue (not unusual). So I would probably defer OLCI to say 90 minutes to departure, and if it was clear she wasn't coming then get her to cancel if possible to perhaps get a bit of money back. If this was a BA Holidays booking then I'm not sure if taxes are refundable or on what basis, but there is no harm in cancelling anyway, nor would I expect your remaining travel to be affected. I think I did hear of one obscure case where the hotel were miffed not to get the expected number of visitors but even that was a bit of a one-off.
MIA is Terminal 3, so Positive Boarding rather than Ready to Fly, but if she isn't at the gate at the right time, the agent will see there was no PB entry and so would allocate the seat to a standby passenger, and for MIA at this time of year there would be plenty of those.