Thanks WGH - I read that when I first discovered H's affair and was trying to make sense of it. At the time it seemed that the likelihood was that H was experiencing the romantic, "drug-like", escape from his own depression affair, but I was too close to the situation at the time to be sure.
Re-reading it now, I absolutely know that this is the kind of affair H is in. I also suspect that his OW is a "spider woman" of sorts, she has alluded to a serious break down in her R with her F as a child and she claimed that she couldn't leave her previous boyfriend because she couldn't afford to live somewhere on her own despite them "hating each other", but very surprisingly engineered a situation where she was able to leave him when H offered her an apartment to live in with him!