Hi!
It entirely depends on what type of experience she wants!
If she wants an upscale, fairly expensive dance club I would recommend
Joy Eslava or
Kapital . The negative side is that there are long lines to get in, drinks are overpriced and it can be quite crowded.
But, having lived in Madrid for a semester, I think that the discotecas are much more fun. Many bars have late night music and the dancing goes on until the wee hours. You can find them in the streets Huertas. The area is full of bars and young spaniards. We walked the streets until we found a bar that was packed with people and had music-always go where the people are!
She will have fun no matter where she goes, the night can lead you in any direction!
Have fun!