I would avoid box store anything because of the "will it actually work" issue...the worst thing you could do as she is getting back into it, is have her always waiting around while the bike is being wrenched on...especially on the ride itself...
look for a used bike...one that might have gone new in the $600-1000 range, but is on sale now...and probably look for something in a hardtail...like a Trek Roscoe 6; Cannondale Trail 5 or 6; Fuji Nevada...something along the lines of those bikes...
and include her in the search. Explain things to her like what suspension is; what a 1x is versus a 2x; why a MTB is possibly better for her, than a cruiser, or road bike things beyond the looks of the bike. So many times, when a SO is being introduced to this activity, they are left out of the decision process. If she says: "I don't care about that stuff, just get me a bike", then you know you can just go...
she might be more initially excited and invested if she is part of that process. Explain the different kinds of riding: singletrack, MUT, enduro, bike packing...explain trail etiquette and possibly some history
even though she is just getting back into it, sometimes knowing the big picture gives a better sense of why getting into it is a cool thing