It seems like one of the things to watch out for is going too deep too soon. Philosophers are a wordy bunch, and for some people, early enthusiasm starts to fizzle after plodding through several hours of Plato's more tedious call-and-response dialogues, or getting halfway through something by Kant only to realize that there's still another 400 pages to go; start brief. :) If the enthusiasm is kindled into a larger fire, then it can start to support those larger works.
Fortunately, philosophy was a popular topic for essayists, and a few early writers were very good at brevity.
The Meditations of Marcus Aurelius are brief and give a good outline of his thought.
Some of Cicero's essays like, "On Friendship" might be worth a look.
Maybe a few essays from Plutarch's massive "Moralia" collection...
Augustine of Hippo's "On Free Choice of the Will" might be interesting if she is curious about Christian philosophy.
Many of Michel de Montaigne's essays are of a philosophical nature, as are Francis Bacon's.
Rene Descartes' Meditations on First Philosophy is fairly brief, as these things go.
If you want conversation-starters, the Maxims of La Rochefoucauld are pretty good -- many of them have the character of witty comebacks, which can give them extra entertainment value.
John Locke's "A Letter Concerning Toleration" might be worthwhile, if that is a subject that interests her.
Maybe something short by David Hume, like one of his Four Dissertations?
Jean-Jacques Rousseau wrote a few discourses on philosophical subjects that might be worth checking out, like his "Discourse on the Origin of Inequality".
Ralph Waldo Emerson's essays are popular, and a good introduction to the American side of philosophy, in my opinion.