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.