So, all we know is B (aged 11) said something to her GF that was rude. And that although she had only meant to be cheeky, she was in fact rude.
We do not know if it was rude noises, a rude word, a phrase, a sentence or a whole joke, so the fact that she repeated the rude thing three times tells us nothing about the length of time it went on for, so it's impossible to know whether B should have been able to notice the stunned silence or not.
What we also know is that GF got angry, B apologised immediately, GF shouted at her, B either removed herself from the shared call, or ended the call for all.
Since then GF has punished both B and her older brother by refusing to accept B's apology (a perceived loss of affection) and withholding an ongoing gift from both children (a real loss of a privilege of something her parents cannot afford to replace.)
DS2 was a "cheeky bugger" like that, at about the same age. We worked hard on teaching DS that we cannot say everything we think without giving consideration to our listeners and that something that may be funny to us could be cruel or rude to someone else. We also worked on impulse control. And that the only thing that is in our power when we have upset someone is the apology we make, but not whether the other person accepts it.
But we also parent with a focus on what is age appropriate behaviour and age appropriate discipline (which does not equal punishment). And that means understanding that 11 year olds are not evil little shits who deserve to be permanently punished for unknowingly being rude.
B has apologised, she has shown contrition. Although GF has imposed a harsh punishment on her already, I would have disciplined B myself for being hurtful in this way (that might be a punishment/a loss of privilege or a sustained effort at working on this type of behaviour or both).
You don't say Createsuser if your DF has taken any other steps than to make B apologise.
This is important as otherwise B will not learn what she needs to learn from this. And because it is the job of B's parents to teach appropriate behaviour. An apology alone is unlikely to resolve the issue.
As for GF. He has IMHO every right to withhold his gift. It doesn't matter that I think this reaction is neither proportionate to the offence nor does it reflect a mature consideration of what is an appropriate reaction given the child's age. (The punishment does not seem to me to take the fact that this is a child into account at all.) He still has every right to do this. Even if all he is now doing is taking revenge on someone who hurt (or humiliated) him.
But if I was the OP's DF, I would now reconsider accepting ongoing presents from the GF. He overstepped the mark in shouting at B. (No member of our wider family shouts at my kids without consequence. At the very least, I would have cut the call myself had my DF started shouting at my child.)
FAOD, I view the child removing herself from GF shouting at her as an appropriate reaction and good self-care.
As for A, I would neither replace the loss of privilege for this child nor would I accept his reasoning. That's because an ongoing punishment is not appropriate for B and giving something to A but not B to make up for GF unfairly publishing both only exacerbates the problem. This would need to be explained to and explored with A to help him understand the issues in play here.
We see every mistake we make, every failure as a learning opportunity. I know from our experience that the punishment must fit the crime, otherwise children will not learn what they need to grow into resilient, self-reliable and strong adults who make good choices for themselves.