Free will absolutely exists as a concept within framework of astrology. Astrology is not a deterministic philosophy or framework. You can influence, change, decrease or increase the impact of a possibility with appropriate actions. The extent to which we use free will to alter our situations varies from person to person, hence the outcomes varies. Also, we actually follow the thought currents in the mind, sometimes acting in sync or against whatever way the mind thinks. Astrology directly deals with the influence on the mind which is seen by planetary movements and co relations basis that on the psychology or mental makeup of a person.
In the horsocope, the free will agent is saturn, moon and the ascendant. When these three are strong, a person uses more of his free will, when these three are weak, the person uses less of free will.