just out of curiosity how do you know that this guy is dating somebody else? Maybe that guy is just letting that girl share his place while his family is away!!!
Another possible thing that this guy had broke up with his wife over the summer and then they decided to get back togther.
I also think the reason why the rates of people who are having affairs here is higher than the west, because I think most westerners or expats in genereal here feels invisible because they do not know that many people here, unlike when they at their own hometowns where many people will recogise them. Because of this, they feel more confident and act more liberal:)