Don't let my flair fool you, for I have strange interest and familiarity with American WWI women's uniforms... And I can tell you right off that's not her uniform! Were she to be wearing her own WWI-era women's volunteer or auxiliary uniform, it would have a big ole no-nonsense wool skirt, and the jacket would be longer, long enough to cover her bottom. There were a few different women's uniforms in circulation at the time, but they're very rare now, some good examples are held in the Smithsonian though! You'll note the army contract doctor uniform is the one that echos the male one she's wearing here.

But this looks like a regular American Army (male) uniform, and looks like she's holding a campaign hat in her right hand there. She's probably just having a little fun in a male friend or relative's uniform. He's a sergeant per the chevrons I believe? Very evocative image though, her grinning in a man's uniform, probably seeing someone off... neat find!