Clearly, they were both at fault.
She was speeding in the left lane, and he wanted to be speeding more.
She passed at least one vehicle in the right lane, and could have pulled over at any time after that – and since she didn’t, that’s on her. Since she noticed the guy tailgating her she made sure to stay where she was while filming that – which she should not have done with a handheld device, and provoke him into tailgating her more – which he should not have done.
He finally passed her on the right, which may be a problem in some jurisdictions – I’m not going to blame him for that – but then he had to prolong the incident by slowing down and speeding up again to stay alongside her and shouting and gesturing, instead of just letting the incident end there. That’s on him again.
Then he “had to” speed up again so that he could be sure to pass the vehicle in front of him (because of his antics) and lost control at that point.
There’s no end of fail in this video, on both of them.
If he had just had a few minutes of patience and dropped back to a safe distance and speed, then he could have passed her at leisure (when he did, in fact), and clear her bad driving and leave her behind. And she could have driven properly, not obstructing the left lane “because she was speeding anyway”, and not become some self-appointed guardian of “this much over the speed limit should be enough for anyone”.
Both of them are terrible drivers.