She clearly overtakes the vehicle. Then undertakes the next moving back into the lane of traffic. In an aggressive manner. It would be permissable if she stayed in the normal lane of traffic (left hand lane).
"The Highway Code discourages undertaking on motorways with some exceptions (rule 268): "Do not overtake on the left or move to a lane on your left to overtake". Undertaking is permitted in congested conditions when frequent lane changing is not recommended. On other roads, the Code advises drivers "should only overtake on the left if the vehicle in front is signalling to turn right" (rule 163). Rule 163 uses advisory wording and "will not, in itself, cause a person to be prosecuted", but may be used in evidence to establishing liability in any court proceedings. On all roads, undertaking is permitted if the vehicles in the lane to the right are queueing and slow moving. Undertaking in an aggressive or reckless manner could be considered Careless Driving or more seriously Dangerous Driving, both of which are legally enforceable offences."
There was no need for her to undertake recklessly.
You can take your foot of the accelerator you know...If she was driving with due care and attention she would have been looking well enough ahead.
That being said the car in question that was sitting in the "middle" (the overtaking lane) lane should have been driving properly too.
You use the left hand lane then move out to the over taking lane to overtake then move back into the normal lane of traffic.