After the mother passed, Lola had nowhere to go, and no job or career or savings. She had nothing, except for the kids that she helped raise since their birth. The author gave her not only a place to live rent-free, but paid her some amount of money he could afford so she could better enjoy her life.
I'm not saying either of our comments are 100% accurate, but you are taking a pretty 1-sided viewpoint to the situation. You can't pretend like the relationship between the author and Lola was strictly one of slave-master - that's disingenuous.
It's really easy to judge someone else for what they didn't do when you have no understanding of what their lives were like.
Also, he clearly didn't plan to take his secret to the grave, it's on the cover of the Atlantic.
I think it's fair to criticize someone for being aware that a person is being enslaved and not doing anything meaningful about it for the same reason that it's fair to criticize onlookers who fail to call 911 on a crime in progress.