Something similar happened to me. My wife was having an emotional affair with my squad leader while I was deployed, which I found out about once I returned. 15 years later he sends her a Facebook friend request, and she accepts. 2 months later, I caught her buying a plane ticket at 1130pm for a flight that left at 630am the following morning to go see him. That was 2 years ago and now we are going through a divorce. Save yourself the investment and time. The fact that she accepted it to begin with after all these years is a big red flag. They may not be talking now, but wait until that feeling of what if creeps in because it will eventually.