It's challenging to make a true postural change—it will take time. The goal is to have your dancer decrease muscle tension and gain spinal strength so it won't take extra effort to stand up straight. I would suggest she explore some fun new types of activity, in addition to dancing, to do this.
Practicing acrobatic skills such as handstands and cartwheels increases both core strength and upper-body strength, which are needed for proper posture. Rebounding on a trampoline can help dancers stand up straight: You can't jump well if you're slouched over. This might go over better than simply doing exercises.
When poor posture becomes a habit, it feels normal. That's why it's essential to bring your dancers' awareness to the muscular tension tied to their posture. For example, using a pinkie ball against the wall, dancers can self-massage tightness between the shoulder blades. Many teachers encourage regular ball work before and/or after class by keeping a basket of balls in the studio. I have seen adults, too, dramatically shift their somatic awareness and improve their posture after receiving some form of bodywork like massage. Muscle tension and poor posture habits go hand in hand.