Ok, let's go by parts, I don't know if you are playing for the first time or if you are returning from an old save (because if it is an old save, this complicates things a little more) but when you go through the restaurant event with alexis, I watched her cheating on you, you will talk to her and you will be given some options:
Say that you saw her cheating on you and say that it's okay for her to do that.
Say you saw it and end the relationship with her.
Or say nothing and ignore the betrayal.
After that, you should invite her on a date to the mall, then she will ask you to buy a 2000 necklace.
After these events, you can ask her out on a date at the cinema, and then depending on your choices, she can betray you in two ways:
The first is if you have said that she might cheat on you, in which case the event will occur normally.
The second is if you have chosen to ignore the betrayal, in this case you will need to have energy below 5 and submission below 50.
I had forgotten that you need to do this event at the mall first, if you have any further questions, several of them can be answered on the wiki website.
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