Wren is Alison’s “Beach Hottie” who gave her the pregnancy scare that summer at Cape May. When she told him she might be pregnant, he became abusive and threatened to kill her. That would explain the flashback where she came crying to Veronica Hastings in the middle of the night with a bloody lip, begging her not to tell anyone. Eventually, he throws the rock at her. Now onto why Ms.D. would cover for Wren… Maybe he was working at Radley around the time of Toby’s mom’s death. He let Ms.D in to visit her, and being she was on the board at Radley no one found her suspicious. She pushed Toby’s mom out the window. Wren and Eddie Lamb know this. She begged him not to tell anyone by bribing him somehow (sending him money, having an affair with him, etc.). So they covered it up by saying it was a “suicide” then an “accident.” This is why Ms.D. covers for Wren when she sees him hit Ali with the rock. He’s covering a massive secret for her. And being that he’s affiliated with the Hastings, Peter forces Melissa to keep her mouth shut about it so something like this couldn’t screw up their perfect reputation.