If your gf already has a 50mm 1.8, I would say buy her a 24mm 2.8;
Canon EF-S 24mm f/2.8 STM Lens Review
It is dirt cheap and very nice for use in a travel situation IMO. It is generically termed a "pancake" lens since it is very shallow and will make for a combined package on her camera that requires very little space in her bag.
It is also far enough away from the 50 mm to serve her purpose of getting more in the frame. It works out to approximately a 35mm lens on what is called a full frame camera. On your gf's camera or most camera's she's likely to own in the future, this is a fairly wide angle lens. Such a focal length would have cost significantly more not that long ago or would have produced far inferior results.
While there are lenses which will provide an even greater width to her view on her present camera, the 24 mm is pretty good for all 'round landscape work and is exceptionally sharp at all positions. To go wider and stay competitive is going to raise your price considerably IMO.
Most shots taken with a wider lens (say, sticking with the 24mm) can be cropped in post production (at the computer) to achieve a more close in appearance without greatly sacrificing image quality.
(You could, as an example, buy your gf a few less expensive but highly useful lenses and spend the balance of your projected budget on higher quality computer software for the purpose of editing and adjusting her shots. )
You might also want to look at the present "kit" lens that comes with most Canon cameras. It is the 18-55mm STM lens;
Canon EF-S 18-55mm f/3.5-5.6 IS STM Lens 8114B002 B&H Photo
Again, pretty inexpensive for its quality. It provides a slightly more wide angle view than does the 24mm though I find both to be useful in any particular situation. These are my "go to" lenses along with the 50mm f1.8.
(For getting very up close to wildlife, I use a superzoom, bridge camera rather than a longer lens on my DSLR body. But that's another thread.)
Both lenses BTW offer a closer "minimum focus distance" which makes the 50mm less desirable for "macro" photography or simply getting in close to a subject to really study the subject in a photo. Neither lens is a true macro but will serve as a generally accepted alternative until you are prepared to spend the truly big bucks.
One thing about Canon is their wealth of lenses which fit so many cameras they have produced over the decades. You can find virtually any Canon lens as a pre-owned copy if you look. IMO buying pre-owned is perfectly acceptable since a lens is probably not going to see a lot of abuse and still be sold through a reputable dealer. Buy refurbished from a trusted source and you may even have a warranty. Canon's own refurbished on line sales site is a good place to begin;
Canon Refurbished Lenses | Canon Online Store
Astro-photography typically implies a lens with some "zoom power" such as;
Canon EF 70-300mm f/4-5.6 IS USM Refurbished | Canon Online Store
So high "mm" value for that lens but look at the "f-stop" value. It too is high which means this is not exactly a good "low light" lens. For photos of the night sky or night time landscapes, you would normally place this lens on a camera which has been mounted onto a sturdy tripod. The camera would have its shutter left open for anywhere from a few seconds to a few minutes and, for this sort of shot, it works well.
Removing the lens/camera from the tripod though leaves you with an affordable lens that isn't great for low light situations when the camera is hand held. To find that lens you'll probably spend your entire budget for both lenses on one lens only and still not have a great low light lens.
Her existing 50mm is a pretty good low light lens. It's maximum aperture setting allows in a good deal of light even when compared to, say, the 24mm. It would be my low light lens for now. Once again, taking one more step towards better low light performance will cost considerably more money and not really turn into an overall more useful lens IMO.
It really comes down to how your gf wants to work. Some photographers carry around bags and bags of gear and have multiple lenses for all situations. Others, many actually since great lenses cost great amounts of cash, have more realistic goals.
It is perfectly possible to carry one camera with one or two very good lenses and do fine on a trip. You'll be less worn out from lugging around heavy camera bags and you'll be more ready when any shot pops up. Hanging several camera bags over your shoulder not only does not make you a professional photographer (many use their smart phones for day to day work nowdays), it does succeed in making you look far more like a target for thieves.
How to Carry Less
How to Photograph the Milky Way (notice the lens used here)
Long zoom lenses get the photographer close to the scene. If your gf wants to shoot images of the moon, that's great but also limiting to other use for the lens. Long lenses also make it more difficult to balance and hold a camera for good, in focus shots if the camera is not tripod mounted. So, how many photos of the moon do you really need?
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