Since no tax is associated with the FBAR/FinCEN 114 form, I'd err on the side of caution and list the highest value for the full calendar year ($100,000) rather than the amount that corresponds with her time as a resident.
There isn't much guidance on this sort of matter aside from some recent litigation that is still not entirely settled (and which is a bit different from the situation you describe, though still dealing with dual status aliens).
There's no harm that comes from reporting the higher value and it'll avoid the potential for any penalty for not reporting correctly.
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