I would say:
- ¡Hola! ¿Qué tal?
- ¡Hola! ¿Como estás?
- ¡Hola! ¿Qué hay?
As it is a friend we speak in an informal language.
Informal language is used when speaking between friends or family. A difference between the formal and informal language is that in the informal the promote "tú" or "vosotros/as" is used.
MissSpanish
