This is not from Roman culture but Classical Greek (458 BCE), but it might be a good point of comparison anyways. In his play Eumenides, Aeschylus uses a word that means “to smile at in greeting (as one might do to a friend).” The Erinyes are chasing Orestes and tracking him by the scent of blood dripping from his hands, and they say, “the scent of mortal blood smiles at me in greeting” (Eum. 253). We can infer from this quotation that there seems to have been a cultural norm of friends smiling at one another in greeting. While this may not have persisted in the Roman world, the Romans were very familiar with Aeschylus, so they certainly would’ve heard of the concept of smiling at friends. Further, since this is not the only instance of this word, we might assume that smiling at friends was common in the Greek world, so the Romans may have been aware of this practice.

As to your question about Mary Beard in the comments, she’s a well-regarded Roman historian, but it is difficult to interpret her quote when it’s used out of context, as you say.