Think about how appalled Edna must have been. How traumatized. How guilty she must have felt over the death of this young girl.
Then realize that Edna anticipated practically every threat that the Incredibles would run into from Syndrome and built help into their suits. The only logical conclusion is that he contacted her–possibly scores of times. Syndrome was a stalkery fanboy before he turned supervillain. And Edna is THE suit maker for supers, as well as Mr. Incredible. Of course Syndrome would go to her. Edna is the best, and Syndrome would want the best designer for his costume.
Think about all that. Think about the woman who was so horrified and grief-stricken by Stratogate’s death being asked by a supervillain–one who was a genuine threat to supers she cared about–to design his costume.
And then realize that, despite her horror and guilt and rage at the gruesome deaths of Stratogate and other supers, despite her vehement conviction that such deaths should never happen again…
…she gave Syndrome a cape.