I feel like Deb gets way more blame than she deserves. She may have known everything, but as far as I remember from the story the mother of the MC said she was the only one against the plan for Marie to disappear. The mother and father knew the whole time, and were visiting regularly enough that they decided to buy a house close by and at the same time were taking the the MC (at least the mother was) as if they were un aware. The mother talked about Deb having changes to her personality over the years after Marie left that would imply it was probably tearing her up inside to lie about it, yet the mother never felt like making it right herself was a good choice. I am sure the nature of the MC and Deb's relationship only makes the "betrayal" worse but I feel like anyone else involved is glossed over (yes MC seems to be upset with his mom but far less so) despite them having much more to do with the "betrayal" than Deb did.
I do not have a save around .08 to go back right at the moment to go through the whole incident again so a lot of the story is fuzzy and I think I glossed over it the last time I played but from what I remember it was only recently (like a few years prior I think) that Deb decided to be back into Milly/Marie's life, all the while the parents of the MC seem to have been communicating enough for the mom to make it sound like the once every 3 months that Deb was involved was not very often. The mother also laid everything out to the MC and bailed without really letting anyone have a chance to to let it sink in and come back for more questions (while also seemingly leaving things out about certain things that had to do with some of the sexual urges.)
I hope the cliffhanger at the end of .09 is more to get the family to come together more and eventually them fully getting over the past now that it has almost killed a second member of their family and not a way to cut Deb out of the story because it seems people are not a huge fan of hers. I feel like she is caught in the middle almost as much as the MC and Milly, between the man she loves more than anything and their family who seems perfectly happy continuing to lie to him and Milly because it was the easy thing to do.
Oh mom and the Ex are equally as guilty or even more and deserve as much flack for sure from MC's pov, just they have not been as present in game as a main as Deb and this betrayal was exactly part of what made her role feel so weird in game as a would be wing woman. This moment was coming all the time.
Just the thing is if Deb would have come out after three months, they might have been able to work it out. Her having kept the secret though for all those years and even after the wife died and keep lying to him all those years makes it such a breach of trust that even if MC could forget his anger over time, found it somehow in himself to forgive her he would not be able to forget. I would not be either in his place. Just getting beyond his first, second and third anger and even more disappointment in her would take MC time though if a human and not just one week
The trust in their relationship is gone even if MC would deep down still love her, whether Deb regrets her choice or not, she kept at her decision anyway and that is a decision with consequences. Just as Mc's decision during that action was one with consequences that could not be undone after.
Now for Deb that means she will have to find a way to deal with this new reality that she was responsible for creating due to her choices. Well if her coming to hate MC will be what it takes for her to move on, so be it. Going back to how their relation was before her betrayal and impact that had on her and MC during those years, let alone back before MC found out will be impossible even if MC is a greater man as I would be and would find it in his heart to forgive her.
Edit: Well if Deb succeeds in her action I suspect it will hurt MC still, despite the betrayal and if she fails and MC will forgive her in her need it is unlikely their relation will work anyway and for the parents , well they will have to deal with the consequences of their smart plan and actions if it means they will lose their daughter, son and granddaughter and same for the Ex if she is still alive and set everything up out of revenge. I hope the results will make her happy. There are no winners in this game as is, maybe MC and Milly will be able to salvage some.