I hope my directness is accepted in the spirit that it's intended.
My two cents would be that we're here in this world to deal with the way things are, and if we don't know how things are, we can't do that. If we decide that others cannot deal with reality, and we have to "protect" them, we are really ripping them off from the opportunity to deal with the way things really are.
If the purpose of my decisions is to minimize upset, then I guess living a lie is preferable. If the purpose of my decisions is "right action" (as the Buddhists say) then I want to be truthful about both good and bad.
Look, he didn't "ruin things" by being truthful. He probably didn't even ruin things by cheating (not that it helped). WIthout trying to attach blame, it's fair to say that the relationship was not in good shape if it was based on concealing truths, regardless of whose fault it was. Healthy relationships don't result in infidelity - it's a symptom (unless you have a sex addict, which it sounds like you don't). She chose how to respond to his confession, and she had a wide range of choices in how to react. She is responsible for her choices there, not him.
It's like a building being ruled as unsafe by the inspector and having signs posted out front...it was unsafe before, but now everyone knows about it.
Love and doughnuts,
The Dood