I think Even was lying about Sonja and him taking a break. Perhaps he was testing the waters with Isak. Before he put anything he has with Sonja in danger.
It’s just that he said, “I told Sonja about you,” not, “us” which doesn’t necessary mean he told his girlfriend about his hook up with another boy. And she just looks so happy with him and enjoying the kisses. Even if she was the most loving understanding person in the world, I can’t see her being ok with her boyfriend wanting to take a break from their relationship because he has feelings for another person, let alone another boy. And then kissing him so blissfully and playfully a few days later after the other relationship fell through.
I think she knows that Isak is gay, but not about the connection with Even.