Legally there are no such things as warning shots. Any shot fired during a confrontation is an attempted use of deadly force. Next question: Did she have a reasonable fear of death or great bodily harm, which is the threshold for self-defense in Florida? No facts here to tell.
If there would be no justification for deadly force, "warning shots" may not be justified either, but I would think there could be situations, especially in cases where deadly force is used for a reason other than imminent risk of harm to the shooter, where a warning shot might be appropriate. As an example, if a person sees someone trying to attack someone with a knife, and it may be hard to shoot the knife-wielding attacker without risk of hitting the victim, but there exists a clear direction where a shot could be fired into the ground without danger. Would it be better to wait in the hope that one gets a clear shot at the knife-wielding attacker before he kills the victim, or fire into the ground in the hope that the sound of the gunshot would dissuade the attacker? I would think the latter course could be safer.
The fact that he allowed her to leave and gave her the opportunity to get her gun, indicates to me that she was not under the level of threat justifying deadly force in self defense.