Lovely the way we can see here how English can use a range of tenses, all for something in the future. After all, Rachel is presumably not giving her the iPhone right at the moment she's speaking.
In situations like this, people will use the present continuous to refer to things in the future where they have decided what they are doing, I think. It doesn't apply to all verbs, but only those where the action is conceptually happening for longer than the physical action takes. People think of the gift-giving process as starting when they buy the gift, if not earlier. It's quite hard to think about where the limits of such constructions fall.
"What are you giving her" here agrees with the previous sentence, but it would be completely natural to mix it with other appropriate tenses/aspects. What will you be giving her, what will you give her, or what are you going to give her would actually all work.
English plays fast and loose with tenses sometimes, including the 'historical present' (which some people consider awful). It can take a long time to be confident about them, because I don't think anyone's come up with a comprehensible set of consistent rules to describe actual usage.