Why does Les say he cannot blame Abraham for listening to Sarah and having a child by Hagar?
He cannot blame Abraham because Abraham waited nearly fifty years after God's promises while nothing happened, making it seem God was doing nothing. In that long delay, taking Sarah's advice and having a child by the slave girl was an understandable human response to unmet expectations.
Les explains that Abraham had waited almost fifty years from the promises first given in Ur. He did not leave Haran until he was 75, and he did not receive Ishmael until near 80. Only when Abraham was 100 and Sarah 90 did God perform and the promises become fulfilled. Les looks at those circumstances and says he cannot blame Abraham for listening to Sarah and having a child by a slave girl, since God seemed not to be doing anything. Les uses this to show how easily faith gets crushed under circumstances, but God's wheels grind slowly yet surely; after fifty years of promises, the child finally came.