I'm the assistant manager at a hair salon/beauty supply store and I have to recommend Moroccan Oil! AMAZING!
We sell a 100mL bottle for $36.95, you might be able to find it cheaper. But it is worth EVERY penny! Biosilk would be my second choice.
A straightening balm or lotion would help too.
Apply a small amount of Moroccan Oil to her wet hair, just from mid-hair shaft to her ends, applying most to her ends. Start with maybe a quarter size then decide if you need more. Then blow dry with a round brush or a paddle brush, it will make a huge difference. Section the hair and do it properly! Using a professional blow dryer will also help smooth the hair eliminate frizz! Then same thing with flat ironing, section the hair and be patient.
When her hair is all straight and wonderful apply a little more Morrocan Oil or Biosilk to her hair, again mid-hair shaft to her ends, applying most at the ends.
Her hair will stay straight until she washes it again, a lot of women leave it for a week!