How can she do this ?

This afternoon on my way to school, l was surprised at seeing one of my acquaintance make a stupid mistake. She bought one piece of cake, and l saw she give the old man five yuan, first the old man gave her nine yuan, then he found maybe he was wrong, and he asked the girl "How much you give me,five or ten?" at that time the nine yuan has given to the girl"ten" "wait, l remember l have 100 yuan" then he counted his money"one two...ten , you give me five yuan, you are a student...you see there only have ten piece and no the eleventh" but the girl replied" how can it do if you've only nine pieces?" then the old man had no words, and the girl turned around and walked away. l don't know why she do so? Maybe she has some thing hard to say, but whatever her behaviour is quite wrong. l am sure she has recognized she's wrong. Because l heard she said"l am sorry" to herself and wiped the tears on her face. l don't know what was happened to her but l just know l should inquire and help her, because l just know she is a good girl.