I blame McDowell, didn't he ever think someone could just be posing as this woman? How stupid can you be, if someone post a message online that says I live at 7827 Walnut Lane and I would like someone to sneak into my house at night and cut my balls off, wouldn't you at least confirm with the person that they wrote that before you agree to do it. For working at a hospital's radiology department, this guy is pretty stupid.
Perhaps he just didn't care and figured if it was someone posing as this woman, he could just blame them for posting the ad.
As a side comment, he must have thought the woman was one hell of an actor. I can imagine him thinking this while he was raping her, "Damn, this bitch really knows how to act out screaming and fighting, if I didn't know any better, I would think she was actually getting raped."