I was told by a friend that there’s a Hadith that says that “Salaam” cannot be said to 3 people: a kafir, a person who drinks alcohol, and a non-Mahram.

Yes, that which your friend has told you is correct. (Ad-Durrul Mukhtaar, Vol. 1, Page 456, Matlab Almawaadhiul Lati yukrahu feehas salaam)

And Allah Ta?ala knows best,

Mufti Muhammad Ashraf
Darul Iftaa - Jameah Mahmoodiyah, Springs

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