I was writing my second BuddyPress tutorial for wpformation.com when i realized it could be helpful for a community admin to restrict private messages to friends only...

imath-pm-friends-only.php

<?php

/*** beginning of the code to paste in your functions.php ***/

function imath_pm_button_only_if_friends( $button ) {
  	if( is_super_admin() )
		return $button;
	
	if( 'is_friend' != friends_check_friendship_status( bp_displayed_user_id(), bp_loggedin_user_id() ) )
		return false;
	else
		return $button;
}

add_filter( 'bp_get_send_message_button', 'imath_pm_button_only_if_friends', 10, 1 );


function imath_fillfield_only_if_friends() {
	
	if( empty( $_GET['r'] ) )
		return;
		
	if( is_super_admin() )
		return;
		
	$path = esc_url( $_SERVER['REQUEST_URI'] );
	$redirect = bp_core_get_root_domain() . $_SERVER['REDIRECT_URL'];
	
	preg_match('/[\?r] *= *["\']?([^"\' ]*)[\& ]/is', $path, $match );

	if( !empty( $match[1] ) ) {
		$user_id = bp_core_get_userid( $match[1] );
		
		if( 'is_friend' != friends_check_friendship_status( $user_id, bp_loggedin_user_id() ) )
			bp_core_redirect( $redirect );
	}

}

add_action( 'messages_screen_compose', 'imath_fillfield_only_if_friends' );

/*** end of the code to paste in your functions.php ***/
?>