<human>: Provide a brief overview of the text.

I will sum this up as succinctly as possible.

At the age of 20, I went on an exchange to another country. I made new friends, including another exchange student. We developed feelings for each other, up until the point that I found out he had a girlfriend in his home country. We had what I now understand to be an emotional affair. 

We kept in contact, and he stayed with his girlfriend. I went to visit he & other friends over there several times, where nothing sexual happened and we maintained our close but awkward friendship.

The last time I went to visit I went to bed, and he came to join me. We decided that we both want to have sex, and so we did. It was... a release of old tension, but otherwise it only made me feel guilty and sad.

I return home and finally understand what I've done (I had had no other romantic relationships in my life up until this point). I know now what I have done to someone else's relationship, and I will have to live with that for the rest of my life. I am not asking for absolution, I just want to be clear that I totally and completely understand what I have done.

My question is this: it has been about 6 months since this has happened, and I am still struggling with it. He lives with his SO (23F), and they have been together for 6 years. The emotional affair took place in the first year they were together (and arguably past that), not to mention the recent sexual contact. He also confessed to me that he'd kissed other women recently, so I know he's not just limited to transgressions with me. 

I want to be clear - I want nothing else to do with him, all I want is to work on how & why I let this happen and I want (more than anything) to learn from what I have done. However, I would like to know if I should tell his SO about what has happened between us. I have read conflicting thoughts on this matter in this sub, and I would like an honest opinion.

Thank you.
<bot>: My 'friend' and I conducted an emotional affair that culminated in a single sexual act. Should I tell his SO about this long-term infidelity?