I can't speak for Jo, but I condone their affair. And I condone Diana's affair with Hewitt, too.
I think it was clear by 1986 that the marriage was over in all but name and that the parties were both desperately unhappy in a way that is probably impossible for us to comprehend. The situation is addressed in some detail by Dimbleby in Chapter 20 of his biography of Charles. At page 394 he writes: "Given the pressures to which they were both subjected by the demands of public life, theirs would have been an exceptionally testing partnership even if they had been able to stumble towards true companionship. However, the Princess's persistent and intense distress combined with his bafflement and exhaustion to make that modest ambition, which they both shared, impossible to realise. Thus, by 1986, their marriage had begun slowly to disintegrate in what were, for both of them, the most excruciating circumstances. That handful of their friends who knew and understood felt only compassion and pity for their shared predicament."
In November 1986 Charles wrote to a friend, "Frequently I feel nowadays that I'm in a kind of cage, pacing up and down in it and longing to be free. How awful incompatibility is, and how dreadfully destructive it can be for the players in this extraordinary drama. It has all the ingredients of a Greek tragedy....I fear I'm going to need a bit of help every now and then for which I feel rather ashamed. All I want to do is to help other people..."
Any other couple would have been able to separate and divorce without the world watching, but these two both thought they were stuck in this Hell for life. They both needed something the other couldn't provide. They both needed friends, and lovers. Charles turned to his old friends whom he had expelled at Diana's behest, and Camilla too. Diana turned to Hewitt for friendship and love. I don't hold the fact of their affairs against either of them.