Reddit mentions of The Claim of Reason: Wittgenstein, Skepticism, Morality, and Tragedy

A book that might interest you and him is Herbert Marcuse's One-Dimensional Man. Marcuse was a Marxist thinker, and he wrote that book as a criticism of what the individual has become in advanced industrial society. He is a clear and entertaining writer, and he has a lot of examples to support his ideas.

A much harder book from a non-Marxist perspective is Stanley Cavell's The Claim of Reason. Cavell is a tricky writer--he's hard to read quickly, and he doesn't have totally organized arguments. Nonetheless, he talks a lot about what it means to be human and what it means to deny one's own or another's humanity. I'd only recommend this book if your partner knows something about philosophy already.

Then there is a thinker like Emmanuel Levinas, who writes about how it is to experience other people. He's also a bit tough to read, but he has a fascinating and highly influential idea of our ethical responsibility to other people. His classic work is Totality and Infinity.

Existentialism talks a great deal about what it is to be human. The thinker Jean-Paul Sartre wrote that there is no human nature, only a human condition. His big book is Being and Nothingness.

The German thinker Hannah Arendt might just be the closest fit to your partner's interest. She wrote a book called The Human Condition that is all about what it means to act.

One last suggestion: it's not quite philosophy per se, but if your partner is interested in technology and media and the effects it has on people, then Marshall McLuhan's Understanding Media: The Extensions of Man might be a good gift. McLuhan is not a hard writer, and he has short chapters. He's a bit of a funny writer though, not only because he makes jokes but because he sometimes makes claims without even an attempt to back them up. However, the book is a blast for someone who is interested in how, say, the electric lightbulb changed human life. Of the books I've mentioned here, it's probably the easiest read.

Hope those suggestions help. By the way, if you could give a sense of your partner's education level it would help. As I said, the Cavell book is probably best for someone who has studied philosophy in depth already. On the other hand, I think a beginner could get into McLuhan and work through Marcuse.