Do I now also need to assert (likesAsFriend Peter Billy)?

If you look up the predicate #$likesAsFriend in the KB Browser, you'll find the following assertion in AgentGMt (although it will be presented in frame and slot notation):

(not 
  (isa likesAsFriend SymmetricBinaryPredicate))

So, #$likesAsFriend is not a symmetric binary predicate. That means that given "(likesAsFriend X Y)" Cyc will not infer "(likesAsFriend Y X)". The rationale may be that Person A may like Person B as a friend, but the feeling may not be mutual. If you want to assert that the feeling is mutual with respect to Peter and Billy, then you will need to assert "(likesAsFriend Peter Billy)."