I never saw any issue with Emily having lunch assuming what she told the MC is the truth and she did not cheat. However I do get why Andres would have an issue with it and yeah she probably should of respected Andres feelings since it's clear she wanted to meet him for her sake not the MC's. But Emily going along with hiding the truth and sending the MC away because he was acting up and she didn't know how to handle him. Well that is a different story for me , as she went along with Andres's ultimatum and to me feels like she didn't try hard enough if her only solution was to send the MC away.
I still think Andres is a bit worst than Emily with or without cheating. Simply because it was his idea to come up with that ultimatum for Emily to never tell the MC the truth. Both are at fault yes since Emily did agree but the fact it was his idea makes him worst to begin with to me despite the negatives for Emily's character. So some see the cheating Andreas did because of patron or they think it's very forced for one reason or another. Me on the other hand it does not surprise me one bit and feels like something he would do if he wants / willing to keep the truth from the MC. Yeah , yeah I get he hates the father and doesn't want him in the MC's life but still not his decision to make. To me that shows he doesn't really care all that much about others feeling only his own. He should of simply walked away if it was such a big deal but he wanted his cake and eat it too.
At least that's how I see things personally I may be off base a bit with my personal opinion but just how I see things. I'm probably forgetting some details but from what I do recall I dislike Andres more than Emily.
Hoisting someone else's child on someone who is madly deeply in love with you is far worse than "I'll agree on the condition that I will be the child's father". Cheating would also be fully out of character for a person who in spite of being cheated on (I don't recall exactly if this was framed as clear cheating or ambiguous cheating), decides to raise a child as his own.
I'll ignore the later re-framing for this as it might just be nonsensical in the story, because Nyx wanted to prevent backlash from Patreon. I'm still not convinced that's the case, I lean more in the "I needed to make him the bad guy, I didn't realise Emily looked like the asshole in the relationship."
You have someone who is so deeply in love with someone they are willing to raise someone else's child just so they can be with her. In love with someone who either cheated or who was not being forthcoming in juggling two (?) relationships.
This person not only raises the child as his own, but even after breaking up with the mom. Takes in the child with his new fiancée to keep raising him. Again a child that is not his, for a spoilt marriage.
Non of that goes with a person who cheats, let alone is a serial cheater. Someone who decides to cheat on his new fiancée, just before they are going to get married.
It reads entirely as trying to re-frame a moral character as an abhorrent cheater who abandoned the mother of his not-son, someone who would then take in his not-son so he could punch him in the face for discovering his cheating.
The problem isn't that it's impossible for Andres to go from moral to an immoral cheat. There was however no work done to get him from a to z. This wouldn't happen overnight, and it wouldn't happen without Emily also massively mocking up the relationship with Andres.