How do you know it was her mic feeding back?
I wouldn't suspect it feeding back in her primary wedges, maybe the sidefills... (judging by the video clip)
What I would suspect (since the rest of the band is on wedges), and this happened on an intro to a song, is that an acoustic instrument was unmuted when it shouldn't have been, and started resonating.
I just finished an acoustic tour leg where 90% of the time, it was the dobro feeding back in it's wedge due to incorrect pedal presses (the player kept the dobro inches away from the wedge, compared to his playing position a few feet away). A gate works well until the rest of the band starts playing, then it opens up and starts the whole feedback process. And hacking that much lo mid out of the strip made it sound too thin to the artists and myself.
The other 9% was FOH related (proximity to speaker stacks with very resonant instruments).
The last 1% was zingers off of background vocal mics due to temperature and humidity changes throughout the show, or in some cases, the stage ceiling geometry.
BRad