We might also note that RMU has now formalized rules for this, which might serve as a guide -- which is a pretty good indication that other people have had this question in the past, Jengada!
RMU has devised a new skill for precisely this: Protect. I'm not sure if RM2 ever had anything like that, but I haven't gone through all the companions to find out. Unless there is something there, I think Peter's interpretation of the Rules As Written is accurate. Personally though, I would probably houserule it to allow the Rogue to parry the attack at a significant minus. For comparison's sake, the RMU penalties are: -25 if the protector is between the attacker and target, -100 if the attacker is between the protector and the target; and -50 in all other scenarios. Skill in Protect will reduce these penalties.