And after the affair, her husband will still be fat, bald, lazy, unmotivated, stupid, sedentary, and as limp as a dead snake. Correct me if I am wrong, but I do not think she promised to love and cherish him as long as he stayed thin, kept his hair, ran two miles a day, read the Wall Street Journal and banged her like a horny bull. Shacking up with another man is not the way to show respect and compassion for one's husband. It is pure selfishness. If the marriage is truly dead, then divorce is the only option.
Most flagging marriages are fixable, though.
Especially if her kids hear about it. They will hate her for it. The last few years of my marriage was hell. Not only was there no sex for five years, there was no affection whatsoever. In fact, I waited until the divorce was final before starting to date again.