This sort of thing happens all the time. It was worse for my Great aunt.....she couldn't make her date with her BF so she asked a close friend to take her place. That night her friend and BF hit it off, and ran off together. Later they were married. So this nothing new. There is no explaining why infatuation/love makes us do these things.
No one is calling you a bad guy, hey bad things happen to good people too. The thing is you two were not married
, no had no proof of her cheating, and it's plain to see that something about your friend really captured her heart. So ya it sucks, BUT you never know you just may find yourself in her shoes one day.