Both the novella and the movie tell us that Alma worked for Monroe while still married to Ennis. One scene from the screenplay shows that Monroe was enthused with Alma even then:
95 INT: GROCERY STORE: WYOMING: EARLY EVENING:
ALMA, a little older and a little less skinny, though still just as cute, is clerking at the grocery store. As she rings up groceries, WE SEE MONROE, now a manager, wearing a cheap tie, flirting, smiling at her. ALMA smiles back. The customer leaves. ALMA and MONROE are alone. MONROE opens a box of Junior Mints and eats one.
ALMA
What are you smilin' 'bout?
MONROE
(chewing, smiles even wider)
Nothin'.
ALMA
Gotta be somethin'.
MONROE
Just happy, I guess.
MONROE throws a Junior Mint at Alma.
ALMA
(laughing)
Stop that I
Just then, ENNIS walks into the store. MONROE and ALMA immediately stop their flirting.
ENNIS walks up to ALMA's counter. MONROE hurries back to the office, afraid of ENNIS.
ENNIS
(pointing to the cigarettes behind the counter)
Two packs.
ALMA
(not getting the cigarettes)
Who's watchin' the girls?
ENNIS
They're outside in the truck.
ALMA
(darkens)
I coulda brought home your smokes.
'Sides, it's way past their bedtime.
ENNIS
(grim}
The quicker you give me the damn smokes,
the quicker they'll be home in bed.