Dilemma

Are there traits within your friend that the GC has "inherited"? It sounds to me as if the child is behaving just like your friend, if that's the case little wonder the child behaves that way!
I have several GC, all different characters and personalities but I love them all the same. Surely it's important to engage with children on their level, not our own!
I think this particular grandmother should grown up!

It is the same with your own kids at times. Sometimes one or two of them are more likeable than others ( and this may vary over time) but it is essential to love them all equally.

Treat them equally of course it shouldn’t even be needed to ask
Maybe there’s a reason perhaps she struggles more than the others maybe she has some mental health tendencies she’s her granddaughter maybe she needs more understanding and certainly not less love

I do know of one grandmother who has been in this situation but the gma made an effort to treat the gc equally.

If these were my children and gma wanted to exclude one child, they wouldn't be having a relationship with any of my children. It's unfortunate but favouritism is so damaging to all children involved.

Agree with hithere, treat these children equally. As much as she might be a pain, she is still just a child who is learning and testing boundaries.

Dangerous territory, she doesnt want to be accused of favouritism.

I think she should try to be as engaging with this gc as she is with the rest.
If gc and parents choose not to reciprocate, she has done all she can.