You should always test ALL your breeding stock in a breed with frame. The stallion test could have been mixed up at the lab or some other such stuff (not suggesting this is the case here, just a "what if" scenario).
Also, if this mare was tested frame and came back negative, I would be very much surprised. She CLEARLY has the phenotype, and with that solid nose, I doubt it is being caused by splash.
She'll get tested prior to breeding just because there are overo's on her moms side, but I did speak to her fathers owner and she told me he is negative so that was a relief. I already got the paper work from UC Davis just have to send in the hair.