The most useful route in my view would be to try to nail down the exact status of a police misconduct hearing. My thought is that it is heard in public, so may have a similar status to an employment tribunal (which can compel witnesses)
I had a quick glance at the 2012 police conduct regulations which cover hearings, and while officers can be ordered to attend (obviously), for witnesses there is no mention of compulsion there, only that the chair can
cause the witness to be given notice that his attendance is necessary
However, there is is provision in law for a higher court to issue a summons on behalf of an ‘inferior court or tribunal’ which would then compel a witness to attend. Again, not sure of the status of your hearing in this regard. But applying to the High Court seems like a lot of trouble unless your friend is pivotal to the case against this officer, which might explain why they are trying to convince him/her to turn up without a struggle.
Seems to me that if your friend is desperate not to attend, then a wait and see approach would work best. If they have the statutory powers, they will use them. If they have to ‘go upstairs’, then they will either not bother, or the letter will eventually drop on the mat. I don’t see that there is any sanction that can be applied for simply asking a public body to do things by the book.
IANAL, obvs