We agree this would be useful but it was not our aim to look at interactions. We also believe interactions such as suggested are better done in longitudinal studies where one can examine moderators.
Second, the authors do not mention why they implicitly selected an additive model (i.e., each factor makes a unique and independent contribution) instead of, for example, an interactional model (i.e., peer and parent factors potentiate the link between individual factors and subtypes of relational aggression).