I think it would be a very bad game if someone have to have real life gardening knowledge to play a gardening sim game hahaha :( I prefer to believe that your girlfriend have a profound sensitivity to intuit the unspokens. That's why she knows how to take care of plants very very well in real life. Plants can't talk, unlike people, they can't come to us saying hey let's talk when they are drowning or in desperate need of water. That's also why she play this game so well, with me failed to write a proper manual.
We do have some visual clue about the light level though. Aside from the curtains above (no curtain vs. thin curtain vs. thick curtain), when we place the plant in the darkest place, under the thickest curtain, we'll see the whole plant become a little bit darker, as if it's in the shadow. But I guess it's not visually obvious enough so people are very likely to miss it. Maybe change it much more darker? And I guess it's probably better if we make the other two places' sunlight level visually different as well. And maybe add some direct descriptions, like different numbers of water icons on each place with different sunlight value, showing they comsume different amount of water.
Besides places with different sunlight value, every changing visual element in this game also matters to the soil moisture thus affect a plant's survival: the weather, the pot, and the soil formula. I think it's probably everything's all mixed up at once at the beginning so it's hard for people to figure out which one causes what. I'm thinking about maybe lock some choices at first, unlock them after survive the first winter, but I personally don't quite like the idea of making people have less choices in hands. So maybe learn from some classic games like two point series or so, make a level by level unlock mode, aside from the sandbox mode?