Unless the matter has reached court, then sub judice won#t apply. Sub Judice means 'beneath a judge' i.e. being considered by a court.
Your supervisor has no right whatsoever to tell you what he/she has said.
Even if it WAS before a court, your friend is your friend, and probably needs you now more than ever.
Ignore what you've been told. If you are not directly involved in the case or the allegation, it has nothing to do with anyone who you work with.
If the staff at work get stroppy about it, seek advice.