This brings a story to mind: My German "uncle" had a friend he traveled with who knew Salvador Dalí. They were driving in Spain and the friend decided they should pay an unannounced visit. Gala, Dalí's wife answered the door and said he was very sick and couldn't see them. When the were back in the car, the artist friend says, "I guess they sent some painting off to a gallery to be sold." The value of art always goes up when an artist dies and Gala was famous for circulating rumors of her husband's impending death to drive up the price of art.
The widow and children of deceased artists often make a living selling and buying the paintings of their spouse or parent. They know better than anyone else the whereabouts and ownership of various paintings, who is collecting, and what prices were paid. They might have an inventory of paintings and it is to their benefit to have fewer authentic paintings out there, to drive up the value of their own paintings. So Maya Picasso has a vested interest in declaring that drawing to be inauthentic, for herself and for her clients and friends.
Maya was born in 1935, probably 30 to 35 after the drawing was done and probably long after it left her father's hand. It's extremely unlikely that she would have ever seen it. Did she give any reason for her judgement as to why it was a forgery? Does someone become an expert on a subject just by saying yes or no, but without providing any intelligent reason?
There are people who authenticate artworks or declare them forgeries, or give their best assessment. They look at the brush and pencil strokes, at the materials (paper, canvas, paint, etc.) used. At how it's aged. Did Lisa ever consult with one of these people, either before selling it, or after receiving Maya's letter? If she would have had the painting insured for authenticity, then she would have had to have had this done. But she didn't want to spend the money and assumed the risk herself. So it should be 100% her liability.