If your friend has a permanent mute, they can appeal it via the account status section of the website.
If that appeal has been used and denied, then Jagex has made the decision to leave the account muted after they reviewed the evidence once more, and will not be reversing their decision.
Outside of appealing the mute, there is no additional assistance that can be provided with regards to this on the forums.
07-Jun-2020 04:16:21