I have a coworker who is a traditional liberal now pretty conservative who is starting to really question things. He's talking a lot about how people seem to be getting stupid and Universities don't teach smart things anymore. He read a book called Coddling the American Mind. I looked into the book and it seems to scratch the surface.
In my opinion the problem at the end of the day is equality. Cultural Marxism aka Critical Theory pushes equality and these concepts have been brought into our education along with feminism which has created an education system no longer about promoting superiority in knowledge and expertise but in promoted equality which of course lowers the quality of everything. Are there any good books that highlight the promotion of equality as the problem with "dumbing down Americans" and how this view of equality has been pushed by the rulers of society for the last century leading to the situation we're in now. Could also tie into how wealth and abundance leads to these scenarios etc... any good books on the subject you'd recommend?
Against Democracy and Equality by Tomislav Tunic adresses the problem of egalitarianism directly. Also, Revolt Against the Modern World by Julius Evola.
But, I must warn you, these books are based on european far right movements that are very different from american conservative tradition, maybe this will be too much for your ex-liberal friend.
Olavo de Carvalho is more sympathetic to americanism and he made a great book called The USA and The New World Order (easy to find on z-library), in wich he debates with russian ideologist Alexander Dugin and discuss many aspects of the influence cultural marxism in america. It is also great book because the debate happened 15 years before the invasion of Ukraine and both authors would become very relevant a decade later.