Should I tell my friend that his boyfriend has an affair?

Hello everybody, I have the following Problem: We invited my friend and her boyfriend to dinner las weekend. She is a very good friend, we used to hang out a lot. After dinner his boyfriend asked me to check his emails on my computer and I said "sure". He went to the computer in the other room. What he did not know is that I always have "Wolfeye Keylogger" installed on my computer, the monitoring software from CNET, that secretely records every password and chat conversation that is typed on my computer. That is how I was shocked to learn, that he actually did log into Facebook to send a message to a woman. You could learn from the chat history that the woman was someone he is having an affair with. Now I dont know if I should tell my friend this story. On one hand nobody likes bringing bad messages. Officially I should not know the truth and the method I used to get the information is not ok... On the other hand I would expect from my friend that she tells me, if she were to know something about my boyfriend. I dont know what to do, can you give me advice? Thank you

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