I think she felt guilty and has been debating over this for a while and eventually decided to tell her BFF. Honestly when someone cheats, its the worst offense in a marriage and to share it with anyone outside of yourself is not an easy thing to do. Also this friend knows OP went through this situation and was probably afraid and nervous about confiding with her, because she feared that the OP would tell her husband.
There could be many reasons, but the BFF eventually confided in the OP to possibly give her direction and advice on how to either move on, or confront. Abandoning the friend does not offer the benefit of getting or to tell her husband. I think if OP continues being friends and continues pushing and dicussing how morally wrong it is, eventually the BFF will crack as the OP stated, she is a good person.
Now with the OP out of her life, the opportunity to let this air out may never happen and the BFF will take it to the grave, and its a total lose-lose situation.
Being friends with someone for 20+ years is not something you just drop because they broke your moral code. Clearly moral codes are broken all the time throughout a friendship, but its the bond that keeps you together and the help to make each other better that keeps you going.
Also the BFF knows her husband way better then the OP and knows the end result will be divorce, cause her husband sounds like me, I don't look back, you cheat, your gone.
OP what would you like the outcome of this situation to be, knowing that your friend's husband will leave her once she tells him? She will be devastated as will her husband, what do you want for your friend?
I am glad that to you it is so black and white. It is not as simplistic a picture as you are painting. The OP is friends with the BH as well. Should she just be expected to abandon and betray that friendship for the BFF, as she was put into such a horrible situation by the BFF? Effectively if she tells, she betrays the BFF, doesn't tell and betrays the other friend (the BH in this case).
Marriage is a more intense relationship than a BFF, yet the friend chose the BFF to tell and not the BH. Effectively the BFF chose the friendship over the marriage (the lesser of the two involved relationships ,I don't care how strong and many years a BFF exists, it never has the bond of a marriage) to spill the details to. Why would it be so bad if the OP chose the friendship of the BH and OP to divulge such information, this would be the same betrayal as the step down from marriage to BFF, going from BFF to friend.
Nobody knows how they will react in the situation of infidelity until face with it. It is easy to yell from your mountain what you would accept and how you would react, but that isn't always the case when faced with such situations (the majority of the time it is the exact opposite). When finances, kids, family, real estate, businesses, and a host of other things come to light and are facing being destroyed, the situational view and outcome can change.
The questions pondered are not fair, as the OP is not the responsible party for a marriage failure or friendship failure if she tells or doesn't tell. Let's not shoot the messenger already. The friend is the one that made the choices and did the actions and none of these are the fault of the OP in any way shape or form. If it works out, it is because the BFF comes to their senses and realizes how wrong they were and it is time to take responsibility for their actions. Just telling the BFF was not a consequence, only a way to burden someone already saddled with infidelity in their life.