Yeah, it sounds like the wife didn't have much grasp of the finances (whether by her choice or because the husband kept his accounts privately), so it's hard to know.
I can imagine a scenario where the husband expected money wasn't going to be an issue long term because the wife was making so much money ($1M though it isn't clear over how many years) doing something she enjoyed, while the wife expected money wasn't going to be an issue long term because the husband had left the company with such a big chunk of money and there never seemed to be a shortage of it when it came time to pay bills. So the husband covers unsustainable expenses out of his savings while waiting for his wife to sell her next novel and the wife (potentially) doesn't feel a lot of pressure to get that next novel sold because her husband clearly has enough money to support their lifestyle indefinitely... until the money runs out suddenly the wheels come off.
In this scenario the main failure is one of failure to communicate about expectations and goals.
But I'm sure countless other scenarios could also explain the limited information given in the letter just as well.