Where is the integrity in behaving one way while no one's watching and holding appearances and wearing the mask of innocence when in the presence of others? It is complicated to act one way and appear to be the other. Betrayal by, with, or upon one's best friend is extremely tough...and certainly brings tons of baggage and no way to check it at the gate.
I could not place myself in that position. Much too heavy.
That goes for lots of people involved here. Certainly BC and NC.
Much of their circle were hitting the seven year itch and many of their marriages were on the rocks. Really quite common. There is a second danger period approaching age 40 that seems to involve people who had married younger and whose marriages last 15+ years.
IMHO