For a long time, I didn't use Tarot cards at all...they sort of lost my interest, I suppose, and other issues became more important. Then I began learning astrology, and now hope to incorporate that into Tarot to expand my knowledge of it once again.
I think Astrology can be used to expand your knowledge of Tarot, but not the other way around since Tarot has no relevance to Astrology. Certain cards are ruled by certain planets, which can aid in interpretation (especially in those nasty Court Cards!). However, you can't use Tarot cards to add to Astrology, unless you're really creative with it.
I would say learn both at first. Learning basic Sun Sign astrology (if you don't know what that is, Sun Signs are based on your birthday: i.e., October 2nd would be a Libra) never hurt anyone and just that has helped me deal with people and learn about them. And just knowing that you can apply it to Tarot, especially in the Court Cards. It's easier in interpretation to know that the Queen of Wands is a Leo, for example, and also helps in imagining a person's physical characteristics: the Lion Queen of Wands would probably have a flowing "mane" of hair, for example. It also helps to know that Leo rules the card Strength: willpower and energy, but also the possibility of being tyrannical in power. Unfortunately, regarding Court Cards, the QoW is the only particular example this website gives that I could find so far.
Overall, start to learn a little bit of both and see which your friend likes best. Keep in mind that Tarot is a little more in-depth but quicker to learn, while Astrology is both logical and harder to learn, just like Dave said.
Hope my advice helped!
~Nicky