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A friend is using the sentence, “Nobody was packing there,” in an historical novel set in the 1885-90 timeframe.

I suspect “packing” was not used in this slang format until 30-40 years later?

Does anyone have corroboration?

1898:

He passed in between his horse and that of his son's, "and got on the
west side of my son, and said to him: 'You are packing a gun for me,
and now, then, God damn you, use it' " At this, witness wheeled his
horse, and jumped off.

The Southwestern Reporter, June 27 -- August 22, 1898

Ngram finds one earlier use of "packing a gun", but it appears to refer to loading a cannon.

from Etymonline.com pack

Sense of "to carry or convey in a pack" (1805) led to general sense of
"to carry in any manner;" hence to pack heat "carry a gun,"

and

1865 Atlantic Monthly One of the best of Rosecrans's scouts..lost
his life because he would..‘pack’ (carry) his gun.

Packing meaning to carry a firearm is a shortened form of packing a gun/pistol etc., which emerges in the Western and Southwestern states and territories at least by the 1870s:

We should be glad to see William out of his troubles but he must quit “packing a pistol” and playing fighter. — Carson Daily Appeal (Carson City NV), 14 May 1873.

Q. The other men who came up, what did they come up for? —A. I didn’t see them until they got right up there.

Q. Did they have anything in their hands? —A. They were packing guns. — Modoc War, US House of Representatives, Message from the President [Grant], Washington DC, 1874.

The quotation marks around the phrase in the Nevada newspaper suggests a fairly new usage.

I suspect that the shortened form, i. e., with no direct object, is a late 20th c. innovation:

The brothers were packing, but that was not unusual; the Party was under surveillance at the time because of the fear of us starting trouble in Oakland after Dr. King's death … — Earl Anthony, Picking Up the Gun: a Report on the Black Panthers, 1970, 107.

Your friend is safe using pack a pistol etc., but it would be wise not to use the verb without a direct object for a narrative set in the late 19th c.

The earliest instance related to "packing a gun" that I've been able to find is this one, from a poem titled "St. Valentines day," in the [Springfield] Illinois Journal (February 16, 1852):

Then the second notion was, to save so much runnin / Arter the gals, which 'pays'—about as well as 'gunnin'— / Which don't pay at all, if you never tried it, take my word for it; / For, in my day, I've 'packed' a gun until I fairly abhor it.

I agree with KarlG's conclusion that "packing"—without "a gun," "a pistol," or "a weapon"—in the sense of carrying a firearm, and in particular a handgun—is a much later development. I suspect that his discovery of a 1970 instance of such usage will be hard to beat.