Ask a woman how many guys she's slept with multiply it by 2. Ask a man how many women he's slept with divide it by 2 then you'll get the real answer. (I'm not accusing your wife of sleeping with the guy though) They met 8 times in two weeks? That's almost every other day. Why the desire to meet each other so often? I don't mean to add fuel to the fire but people who have affairs don't meet up to "have coffee" especially if they have been amorous in their text messages. That just builds up the heat in my opinion. So just imagine what happens when they see each other in person. The mere fact that they met each other means that they were ready to take it to the next level.
I still believe in saving the marriage though. I guess you just have to deal with your wife's character. Hopefully she'll have a change of heart.