well, it's pretty simple.. even if the two of them kept it a secret from the boyfriend.. the fact remains that the other one cheated.. normally, this would be a source of guilt or something that will always stab the conscience of the one who slept with someone other than his lover.. that is if he really love his partner.. try to put yourself in his shoes..if you love your partner wouldn't you be eaten by the guilt from inside out?? unless, you're the type who don't care for this things..
so, to answer your question, it's simple logic that betrayal of trust could destroy a relationship.. “For there is nothing covered that will not be revealed, neither anything secret that will not be known.”
People's logic really astounds me sometimes. I mean..how can you break up a couple if you sleep with the boyfriend of the one you love and keeping it a secret??? Whaa???