As far as I understood the timeline is,
- Emily dates Santiago on and off through high school and university
- Emily got pregnant to MC
- Emily breaks up with Santiago
- Emily starts dating with Andres
- Emily met Melissa and breaks up with Andres
- Emily goes to a bar with Melissa, Santiago still thinks Emily and Andres dating
- Andres apologise and probably they start dating again
- Emily tells about pregnancy to Santiago (first meeting with him after breakup probably the last too)
- Santiago's parents offer money to Emily for abortion
- Santiago tries to talk Emily at the beach, she runs away.
- Emily gives school a break because she is pregnant
- Santiago graduates and their connection lost at this point
- MC born and Emily returns to school
- Emily and Santiago reconnects when MC around 12 years old
In the photo Emily and Melissa looks young so I don't think this is from after reconnection. Photo has Melissa in it which means after breakup with Andres and Santiago. She does not look pregnant which means her pregnancy at most 4 months, she spent at least 1 month with Andres before telling him. Which leaves us 3 months time window tops, in that 3 months as far as she told to MC she only met Santiago 3 times at the bar, where she told the pregnancy, beach none of them were possible to take that picture. If the photo is not a fuck up by Nyx, Emily cheated on Andres and Melissa was there and absolutely fine with it.