A good example that illustrates moral ambiguity in the character of Biff Loman can be found in Act Two, particularly in the "restaurant scene".
This is the scene where Happy and Biff invite Willy to dinner at Frank's Chop House in order to celebrate Biff's meeting with his former employer, Bill Oliver. This meeting is thought to be a great chance for Biff to obtain a loan that will help him start his dream business.
We learn that the meeting never takes place; Bill Oliver does not even remember Biff, making Biff realize that he was merely a shipping clerk for Oliver, and not a salesman, like Willy had convinced him to believe for years.
The anger that Biff develops after realizing that his life has been a mere fragment of Willy's imagination makes him want to confront his father at the restaurant. This, he does in aims that Willy would stop, for once and for all, creating fantasies about Biff in Biff's own head.
However, Willy has a hallucination in the middle of the argument leading back to when Biff discovered that Willy had an affair with "The Woman". This hallucination confuses both Biff and Happy. Hence, Biff takes off and leaves the restaurant in anger while Happy runs after Biff and takes with him Miss Forsythe and Letta- two call-girls whom Happy hoped would have a good time with Biff and himself.
The two brothers then leave their father at his own mercy in the middle of the hallucination, and do not show up until much later at their parents' house. Happy brings flowers to try to make up for what they do, which are rejected flatly by Linda.
This scene shows moral ambiguity in two ways: first, Biff and Happy do not have any issues with taking off with call girls. If Biff were morally ambiguous, he would have made a distinction between going out with a nice woman, or not going out at all. However, he chooses to follow his younger brother's lead and spends the night with these escorts. A man with a set of moral values would not have done this.
Second, Biff leaves the restaurant with no consideration for the age and mental condition of his father. Biff is too much into his own issues to understand that, regardless, his father has lost more in terms of time, life, family, and money, than Biff will ever lose.
For these reasons, it is safe to conclude that Biff is not a morally-sound character. He has no problem with dealing with escorts, and he does not take real responsibility about disregarding the needs of an aging parent. No matter how badly Willy may have raised him, Biff is still an adult. In his case, he is the adult who could very well bury the hatchet between he and his father, and move on the way normal people are supposed to do.