That's good to hear, but as a practical matter, with the symptoms she displays, I'd keep assuming she's positive for care and quarantine purposes. At her age, she'll do fine. False negatives are not uncommon with many COVID-19 tests.
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ex: 2004 Thermonuclear Yellow RX-8|6MT
ex: 2015 Estoril Blue M235i |6MT|slicktop|MP exhaust - PCD
2020 Sunset Orange M2C|6MT|Black 788s
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