The "fact" that either the Harringtons or LE has/had asked the friends to remain quiet has not been confirmed by any one in an official position. A "friend" (who posts on the FM website) of the friends says she/he was told by the friends that Dr. Harrington had requested they remain quiet.
IMHO if this were true, Dr. Harrington (who has been asked the question on the FM forum) would have made a statement to that effect. By doing that he would have removed a lot of "heat" the friends have received as a result of their silence.
I believe the friends silence (not assuming any guilt or innocence here) is as a result of lawyers the friends have hired.
It seems to me that if a lawyers was to advise a client to be silent the first and most important people for them to be silent to would be the police, but by all accounts the frineds have cooperated fully with LE.