Lighting, honestly, I'd say that's what people are missing the most once they've got the basics covered. It's not the most glamorous thing, but it makes a brutal difference in how the stream looks; a decent ring light or a two-piece LED panel fits just right in that budget range. The thing is, after spending time in front of the camera with poor lighting, people get tired of looking bad on screen, even if they don't always say it outright. If she streams or does live broadcasts regularly, some RGB lights that look professional but also let her play without her eyes hurting are pretty much essential.
Another solid option is a good desk or monitor arm if her current setup is kind of improvised. But honestly, in my experience watching friends who stream, what most of them end up being grateful for is something tangible they use all the time. A phone or tablet holder if she occasionally uses it for chat, or some quality monitoring headphones so she can hear her own audio better while she's streaming, are investments she'll notice right away.
If you want something more creative, a portable green screen also fits in that budget and lots of streamers find it useful to look more professional without taking up much space. What I wouldn't do is spend money on software or subscriptions; better to get something physical that she sees there every day in her setup.