I cannot test it right now, but it should work like this:
add_filter( 'bbp_get_forum_freshness_link', 'wpse_77441_change_time_format', 10, 2 );
function wpse_77441_change_time_format( $anchor, $forum_id )
{
$last_active = get_post_meta( $forum_id, '_bbp_last_active_time', true );
if ( empty( $last_active ) ) {
$reply_id = bbp_get_forum_last_reply_id( $forum_id );
if ( !empty( $reply_id ) ) {
$last_active = get_post_field( 'post_date', $reply_id );
} else {
$topic_id = bbp_get_forum_last_topic_id( $forum_id );
if ( !empty( $topic_id ) ) {
$last_active = bbp_get_topic_last_active_time( $topic_id );
}
}
}
$date = bbp_convert_date( $last_active );
$time_since = bbp_get_forum_last_active_time( $forum_id );
return str_replace( "$time_since</a>", "$date</a>", $anchor );
}