If he has actually no background in western philosophy I'm not sure that sending him down any laundry list of 'classics' is going to help him much. None of these people worked in a vacuum and understanding their times and influences is important. He'd be vastly better off sticking exclusively, at first at least, to secondary lit that explicates the ideas of these philosophers in the context of their times and lives, rather than just going straight to the works themselves.
If you do insist on a list however, I'd say;
[Secondary book explaining Greek religion and the context of the first philosophers]
Plato - Apology, Euthyphro, Crito, Symposium, Phaedo, and Republic
Aristotle. - Ethics, Politics, The Organon, Physics, On The Soul, and Metaphysics. + Aristotle by Christopher Shields (2nd Ed.)
Hobbes - Leviathan (abridged)
Descartes - Discourse on Method, Meditations of First Philosophy
Leibniz - Monodology
Spinoza - Ethics
Hume - Inquiry into Human Understanding
Kant - [Dont even bother at first, just get secondary lit. Kant by Paul Guyer, or Starting With Kant, Kant for The Perplexed]
then Prolegomena, CoPureR, Groundwork on the Metaphysics of Morals, CoPracticalR, then CoJudgment. Seriously, if you are going to do a deep dive on anybody, him and Aristotle are the only worthwhile two.
-German Philosophy: The Legacy of Idealism
Hegel [actually just stick with secondary lit, you aren't smart enough to read Hegel]
Marx - The Marx/Engles Reader, just read that and Capital (Abridged). Also David Harvey's book about reading Capital.
[Understanding Marxism by Geoff Boucher]
Schopenhauer - WaWaR
Kierkegaard - Either/Or, Fear and Trembling
Nietzsche - Birth of Tragedy, Beyond Good and Evil, Genology of Morals, + The Portable Nietzche compilation and Nietzche by Walter Kauffman
[Darwin by Tim Lewens]
Husserl [Husserl by David Smith or The Philosophy of Husserl by Hopkins]
Heidegger [On Heidegger's Being and Time by Critchley and Schurmann]
Merleau-Ponty - Phenomenology of Perception
Sartre - Being and Nothingness, Existentialism is a Humanism
De Beauvoir - Second Sex
[Existentialism: From Dosteovsky to Sartre by Kaufmann]
[Freud by Johnathan Lear]
[The Bloomsbury Companion to Analytic Philosophy and Analytic Philosophy: An Anthology]
[Understanding Poststructuralism by James Williams]
Althusser - For Marx, On Ideology
Foucault - History of Sexuality, Discipline and Punish
Derrida - Of Grammarology
[A Critical Introduction to Lacanian Psychoanalysis by Bruce Fink]
D&G - Cap & Schiz/Anti-Oedipus
Zizek - On The Sublime Object of Ideology
[Speculative Realism: Problems and Prospects by Peter Gratton]
It's heavier on more recent stuff because I can't say what is going to matter more or less into the future but that's a list that would more or less bring you up to today.