I don't know if the five best friends can be compelled to give evidence, I would doubt it, hopefully some legal expert can clarify for us.
However, the five do seem to be fairly pivotal to the case for both sides - one side wants them to confirm how they knew about the letter and its' contents, and whether or not MM gave them permission, even tacitly, to the interviews. The other side needs them to state they did the interview entirely off their own backs and their 'best/close' friend was totally unaware they were doing this, but how they avoid knowledge of the letter and contents, I don't know.
I think a part of the MoS case appears to be that by dint of the interview these five gave privacy and copyright had already been breached and Thomas Markle had the right of reply. If the five refuse to appear I'm not sure which side will benefit the most, I tend to think their non appearance will weight more in favour of the MoS.
I struggle to believe that these five 'best/close' friends just woke up one day and decided I know what, let's all give an interview to People Magazine about MM and the rift with her dad and we can tell the world about the letter she wrote to her dad, this took planning and organising and I can't see them going ahead with the interview without first making MM aware and seeking approval. Imo they did it at MM's request. The daft thing is the furore around Thomas Markle and the wedding no show was dying down, interest had waned, it wasn't a big story anymore and all this interview did was stoke the embers back up into a fire and prompt Thomas Markle to retaliate, it was ill judged and ill timed.
I bet the five are totally pissed off, they are damned either way and whatever relationship they did have with MM will be under great strain now or even finished.
As DeRigeurMotis says, this court case, whatever the outcome, is going to be detrimental to the H&M brand.