A hormonal imbalance, only for her there's no cure, added to a love that never went too deep. It's a dead end for David.
Even if he can prove it, the marriage is already damaged. How can David prove this? Anna has already made her decision.
After your post, I think the only way to keep something dirty running would be for Anna to find out the truth about David, otherwise we're going to see a fuckfest with David divorced or in the background

But what would it mean for David to disappear from the picture? Does Anna really see herself raising Sophia alone? Many of the lowlife around her want to fuck her, nothing more. And Kendra's the one who's setting her up to go down.
On the other hand, I know sons of small/medium business trying to survive. The "dreamier" sons are driven out, and the sons who get involved become ruthless. It's their money, it's their future. David is described as a soft guy and that doesn't fit