Why do we never see a two faced person coming? We may have been friends or colleagues for months or even years, but there has never been any indication that this person is going to turn, and turn badly on us.
The trouble seems to be when a third person gets involved. I believe that the relationship between the two people will work as it has to balance even in a slightly crooked way. But add to the ingredients, a third person, then the whole ambiance goes out of the window.
Maybe the third person has a couple of facets of personality more similar to your friend. For example, you trust people, they are less likely to trust someone. Add the third scenario and the first person may feel that they have someone else to back them up.
But why turn on the first friend even though they may have done nothing wrong?