We are probably all familiar with this one: How come Fred and George don't realize the long-dead Peter Pettigrew is alive? They had the map for many years while Wormtail was at Hogwarts (he was Percy's rat first).
It is likely they just thought Peter was a friend of Ron's. Hardly anyone knew the whole story. Even when they saw him sleeping with Ron they probably thought that he just slept in the same dormitory. They wouldn't care about who Ron was friends with! They were some of the greatest troublemakers Hogwarts had ever seen!