I predict that Maddy is going to seek revenge on Nate and not Cassie.
At the wedding, Maddie looks so hurt when she saw them to the point of leaving early, but not because she misses that relationship. When you look back on the seasons prior, Maddy was very much in control in the relationship, something Nate hated, and both of them would step out of the relationship or take breaks. She’s not hung up on him at all. She’s more hurt about the fact that he intentionally tried to sabotage her life by stealing her dimwitted Bestfriend. He could have just left or cheated with anyone but he strategically chose Cassie who he knows it’s easily manipulated by men. This was PERSONAL. Yes Cassie betrayed Maddy, but Maddy seeks power and in her mind, Nate won. Seeing them reminded her of that.
I believe Maddy is saddened by the fact that that relationship is still going on more so because Cassie lacks freedom, not because she misses Nate. Cassie has no identity of her own and has always longed to be like Maddy, but instead, Nate was able to take her and mold her into the version of Maddy that he deemed acceptable. He kept the parts he liked and got rid of the parts he didn’t. Maddy wants to ruin Nate’s life more than Cassie’s; she pittys her and may even still have love for her. Seeing them both at that wedding made her realize who she was looking at (her toxic ex who was very abusive and her Bestfriend who wants to please everyone around her, particularly men) and I believe that’s when it clicked that Cassie was “sort of” a victim because Maddie knew both of these people personally and what they are and aren’t capable of. She’s going to come back into her life and steer her in a direction that she knows Nate can’t handle, thus making him lose his cool, because Nate has been hiding who he really is. He’s a ticking time-bomb who has been able to mask it all because he molded Cassie in the way that doesn’t disturb him. But once Maddy gets back in the picture, he’s going to explode and that’ll drive a huge wedge between them.
In Maddy’s mind she’s actually helping Cassie gain freedom, getting revenge on Nate for stealing her bestfriend who he tried to groom into a trad wife with a sprinkle of her and profiting off of it all at the same time. If she does this she’ll be getting everything she wants.