Mom is not the big fan of Erik's that his father is. She realizes that as a new neighbor just meeting the Costellos, that comment would be bragging and possible set a negative tone in the relationship between the two families.
Paul knows that Erik surrounds himself only with those of a like mind who will be subservient to him. Paul has seen this same situation occur several times in the past. Any friend of Erik's will be dragged into his nasty mind set and will do his bidding. Mike Costello was not like Erik's usual friends. Perhaps that is why he was killed off early in the story. Mike would serve as the shining example of a good kid to contrast with Erik's dark nature.