Curly Frizzy thick hair hep?

Ok i bet myfriend that this yr at camp or at a sleep over i can make her hair very nice and sleek, like mine is (i have loose curls) but i realized this is a hard challange. She has very thick hair like me but her curls or either mushed or verry very tight. When ever she straightens it it just poofs out and dosent stayclose to her head. Is there any adivice any one can offer me i want to help her.I put alot of conditioner in and brush it while in the shower then continue to brush it then put heat spray in and blow dry then straighten. Im not sure this will work on her she has a conair straightner a thin one that goes very hot and i have a thick one that goes equally as hot. PLEASE HELP THANKS SO MUCH