The bottom line is that a keratin smoothing treatment is not a chemical straightener/relaxer. It does not break the structure of the hair as a relaxer does. It weakens the bonds enough to attach new keratin protein molecules to the hair, which is what smooths it out. It is a relatively more natural and healthier process compared to chemical straighteners.
The amount of straightening that occurs in a smoothing treatment is dependent on so many factors. No two people have the exact same hair type, exact same lifestyle, and the same results. Your friend may have had fairly straight hair to begin with, and just needed to reduce the frizz and revive the hair to get to its current sleek state. If your hair is very curly to begin with, then the treatment can reduce frizz, renew shine, and make your hair more manageable, without necessarily making it pin straight.