I do not understand why a PM in this situation would take control at 400ft AGL before landing, surely this is a recipe for disaster?
A sensible, and in the circumstances, reasonable question - but short answer, no it isn't.
Almost all Line and Training Captains take their responsibilities of ensuring FO's are given the maximum opportunities to not only fly to the limits of their abilities, and where safe, to also allow them to learn from their own mistakes. This means on occasions, an ideal descent path or approach may get stuffed up, but error recovery is an important learning aspect too. None of us are perfect, we all make mistakes, thankfully almost all are recoverable, and the only thing dented is a little pride, but along the way another learning point has been made that makes the individual a better/more experienced operator.
The last 1000' of an approach should be stable, but plenty of factors can upset the stability. In the last 100', which is normally covered in less than 10seconds, an on the ball Captain will be in monitoring overdrive when he is PM.
There are many things that I have seen both incompetent or inexperienced pilots do in the latter stages, that left unchecked could easily have returned the aircraft to kit form.
Clearly the closer one is to the ground, normally the more precise, direct and firm ones suggestions or orders become. Similarly static or dynamic weather and wind conditions play a factor in stability and your comfort/recovery zone.
However, there comes a point when a PF may be unable to process the suggestions or orders quick enough, and to try and do so may be counterproductive anyway and overload the PF.
By way of example, I have had to take control at 30' in the flare, at the same time as disabling the FO's inputs which were opposite to mine, on what was an otherwise excellent approach. There's just no time to have a chat about what he should be doing.
Therefore quite simply, any capable and efficient Captain, should know far he can let the FO go and still recover safely, before he takes over and lands.
The art of the equation is knowing the limiting factor of your own or the aircraft limits, to ensure as Captain, you take over before either have gone past a point of no return.
In this case, it would appear that she either let it go too far past her abilities, or she didn't display the correct skillset herself, possibly both. Either way it is a very disappointing read.
That a significant number of FO's would try to avoid flying with her speaks volumes, even if it doesn't pinpoint the why's. It's good the company gave her extra CRM training, perhaps more drastic measures were needed, but this is the luxury of hindsight.
Not sure what her Company culture was like with regard to go-arounds, but I have done many, a small percentage due to pilot error in not achieving a stabilized approach, and I have NEVER been queried by management at all.
If only either pilot had called it for what it was, and shot a second approach...