natural curly hair suddenly straight

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   My sister is 42 years old and has had thick, natural curly hair since birth.  Recently, she had about 2 inches cut off the length and long layers added, which she has done before with great results.  
However, after her last haircut, the hair on the sides of her head in now bone straight - but the back of her hair is still very curly.  I gave her some of my curl products (I have wavy hair and finally found great curling products) - but nothing has worked for her - the sides remain completely straight - not even a slight wave.  She doesn't color her hair, has never   used harsh products and most days she doesn't even blow her hair dry. She has always had thick, curly, healthy hair.  

Does anyone know why this has happened to her hair?  She is very discouraged and doesn't know how to style it anymore - she just wants her curls back.