I remember studying this exact formulation or parties to murder in Crimes 200. I am too lazy to look up the law on it, so this is all just IMO as to general policy and you can always google if you want the precise law.
From a policy perspective party offences evolved to stop this exact type of defence. So for example A and B shoot a man. Then in court say "ha ha you can't prove which one of us pulled the trigger therefore you must find us both not guilty!" It gets more complicated e.g. armed bank robbers where people have different roles e.g the getaway driver - and many countries codified this to make everyone responsible for whatever bad stuff happens.
tldr - the idea of party offences evolved and was codified to punish everyone who was directly responsible, even if they are not the killer. Each can be guilty of murder, even where we don't know which one did the murder (sometimes it is impossible to know).
www.cps.gov.uk
In respect of Girl X, I am sure the prosecution is going to say she had a clear plan to murder the victim, even if she struck no blows and didn't bring her own knife. They will argue exact role does not matter as she was the mastermind.
I think counsel for Girl X will say look this was all a bunch of text messages with a boy she didn't know well, surely she never expected he would actually do it - no one would expect that. He didn't even know the victim. etc. As such she lacks the requisite intention for murder and she never actually foresaw it, despite ostensibly planning it. I'll be interested exactly how counsel tries to argue this.
This may sound strange but I think there is some validity to the idea that neither accused really believed the other was serious, but then according to defences' case one or other crossed the rubicon. There was one of those moments of truth in cross where counsel asked Boy Y something along the lines did he ever stop to think the game had gone too far
But whether that argument would be enough for Girl X with a jury remains to be seen
02c