If you ring DVLA and tell them she has no tax, you must also tell them where the car is parked: eg outside work through the day, or outside her house at night. It must be parked on a road, not on a drive or in a private car park.
They should then come out and clamp it within 3 days.
She will then have to tax it within 24 hrs (which she can't do without an MOT or insurance) so then they will come & tow it away.
I know this is what happens because it happened very recently to my moronic next door neighbour....and yes it was me that grassed him up.