Yeah, Tep, it was utterly fucked up. His moral rigidity his insistence on being as harsh and judgmental toward himself as toward anyone else, just made it all worse.
Everyone who knew them both was convinced that they had had an affair, and everyone was also convinced that it ended after just a couple of months. But Lewis couldn't allow himself to walk away from the mess: he'd promised his friend to take care of his mother, and the fact that he'd subsequently slept with that mother and she'd turned out to be mentally unbalanced didn't free him from that promise; in fact, in his mind, he was now more responsible for her than ever. She was unwell, widowed, without a son, and now fixated on him because he'd sexually used her, so clearly the only thing that could make it all worse than it already was would be to abandon her. So he stayed. He supported her and her daughter financially, said "How high?" every time she told him to jump, and made a conscious decision not to get involved with any other woman while she was alive because, having been such a selfish sex-obsessed bastard, the least he could do was spare her that pain. (And this was even before his conversion; he was a furious atheist at the time, but no less a rigid moralist.)
So seriously fucked-up. Stubborn, emotionally immature, trying so damn hard to do the right thing and be honorable and flailing without a fucking clue.