As Karin and Natalie stated, you’ll want to watch your client’s form through each exercise, then keep breaking the exercise down, until she is able to complete the exercise without compromising that form.
Since this is something she has been dealing with all her life, then her progress will be slow. I always start clients off with feeling what a neutral alignment feels like. Photos of their current posture usually helps clients start to notice how that deviates from a neutral alignment. Then I start with just bringing awareness to one area of the body, usually the feet. Clients often can not focus on changing too many things at once as it becomes overwhelming. Then, like Karin, I work with the stability ball and the mat on basic stability exercises for the core, really having clients focus on maintaining their neutral postures. As to which exercises I choose, that is individual to each client and how their posture lines up.
Good luck! Start small and move slowly. She’s had this posture all her life, so it won’t change overnight.