If you found out your friend was a big cheater, would you still be their friend or tell their partner who isn't your friend?

He has started secret messaging me and bragging about it. I don't know his girlfriend tho. I wonder if I should stay out of it. Part of me likes that he is confiding in me. But I feel disgusted now too. I have never understood cheating.

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Ohhhhh... I made a big mistake and it's very lucky I didn't go running to his girlfriend *phew*...
He had snuck out to mastubate late at night, high, in his car. Not actually cheat and we had crossed wires in messages. Such as me asking if he had just been naughty then... him saying yeah. But I wouldn't class mastubating as naughty!
If you found out your friend was a big cheater, would you still be their friend or tell their partner who isn't your friend?
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