Symbolically, the woman Willy sleeps with in Boston represents Willy's moral failure.
Despite his repeated pronouncements that a person must have character to succeed in life, Willy demonstrates a clear lack of integrity by having an extra-marital affair.
We see a few important elements of Willy's character in this contradiction. First, Willy is a dreamer who does not seem capable of truly dealing with reality. He refuses to face up to what he has done and, instead of asking Biff's forgiveness or Linda's forgiveness, he denies the facts and Biff moves out of the house.
In this denial of the facts of his wrong-doing and moral failure, we see Willy's moral weakness. While he has some talents, some charm, and some other admirable qualities, Willy is essentially incapable of doing difficult moral work. He cannot admit his guilt.
This guilt is symbolized by the woman in Boston.