Satoru met his friend through a mutual friend of yours, and they instantly clicked. You didn't understand it, but their friendship didn't get in the way of your relationship, so nothing to worry about.

Overtime you noticed how she would constantly be clinging onto him, or making him stop his conversation with you so she had his attention. Satoru being oblivious, didn't understand why you two didn't get along.

The two of them are now hanging out at yours and Satoru's place, while you're at work. You knew about this, as Satoru is loyal, but his friend did not. She started trying to make moves on him, and unlucky for her, you just arrived home.