Question: Matty introduces her friend as Mary Ann, but in the year book, the friend is actually Matty Tyler, and she is actually Mary Ann. Were both girls in on the scam? (00:31:22)
raywest
11th Oct 2021
Body Heat (1981)
Answer: It appears that the real Matty Tyler was not initially in on the plan. It's confusing, and there're many plot holes, but it seems the fake "Matty" (Kathleen Turner) intended for the real Matty to eventually discover that her identity was being used (by Turner). The real Matty was then apparently blackmailing fake Matty to keep quiet. It appears that fake Matty intended to lure and then murder the real Matty, framing Ned Racine for her murder, as well as Edmund's. The real Matty's body was identified as being Edmond's wife through her dental records. Fake Matty probably intended for Ned to be killed in the explosion.
Answer: I noticed the problem of the introduction, also. It seemed like a major plot hole to me. There wasn't any material in the movie to support blackmail, etc by the real Matty. I hadn't thought of her possibly being in on the scam. If not, why wouldn't the real Matty have immediately blanched when hearing herself be introduced with the wrong name? So far, I agree with the OP's suggestion.
The real Maddy was at the house when Ned arrived. Presumably, she had already discovered what "Fake Maddy" was up to. It looked like Fake Maddy (Turner) gave the real Maddy a check, presumably a payoff to keep quiet. The real Maddy may or may not have known exactly what Fake Maddy was planning, but went along with being introduced as "Mary Ann." Also, the movie deliberately leaves details vague because it is a big plot twist at the end when Ned, and the audience, learns that Fake Maddy is really Mary Ann.
raywest ★
No. Maddy found out about Mary Ann taking her identity somehow, and I believe she wanted money. When introduced to Ned Racine, she doesn't seem surprised at being introduced as "Mary Ann." She also apparently witnessed the phony will. However, I don't think she knew or had anything to do with the murder. The two women weren't such great friends since the real Mary Ann murdered her.