I doubt she would see a huge benefit in upgrading the lenses she already has. It's not a bad idea though if you think she will stick with the hobby as a premium lens has a long life and can always be used again on a nicer body. But I would try to get a lens that does something she can't do right now.
If you upgrade the lens she already has i would ask if they have ever held her back? Has she not been able to get a photo with her lens and would upgrading it solve that problem?
Another thing that could help is if there is a specific thing likes to photograph, landscapes, flowers, people, indoors, etc. Specific lenses and their features work better for some subjects.
To me image stabilization is important. Especially in a zoom lens. Without it you will be restricted to higher shutter speeds which means you need more light, or you need a tripod. To say it another way image stabilization allows you to get pictures in lower light levels, sometimes substantially lower.
Lighter and better quality dont really go together. As lenses go up in quality they get heavier. More glass, more robust construction, wider apetures all add weight.
Polarizing filters may be a good idea. They are primarily used for landscape or photos with the sky in them. Depending if she edits her photos almost all the other filters can be reproduced with software, usually in a more controlled way.
Mefoto has some good tripods at reasonable prices. I bought the road trip and like it.
I would suggest the canon 10-18 efs lens. It doesnt sound like she has a wide angle lens yet.
Canon also has a 35mm efs macro lens coming out that looks promising if she likes to photograph macro subjects.
For a fast lens the canon 85mm 1.8 would fit your budget. The 50mm 1.8 may be another good option. They are cheap and people love them. I happen not to like my 50mm but most people do. The 1.8 apeture means these lenses can work with much less light. They can typically take photos in a moderately lit room with no flash.
If she doesn't have one yet a nice camera bag is good to have. I just picked up the tenba dna 13 and really like it.
One last idea would be an adobe creative cloud subscription. It's $10 a month and you get lightroom, photoshop, and a bunch of other stuff. Enough tutorials on youtube to learn everything. Editing photos with lightroom will allow her to make far bigger changes than any lens upgrade. The program can be a little complex at first because there is so much you can do. But there is a youtube video to walk you through everything.
Bh photo is where i get most my gear. Their prices usually match amazon and free shipping and no tax. Be sure to check the official canon site for refurbished lenses. They are cheaper and sometimes on sale too. They come with a one year warranty.
My last suggestion, i promise. If she isn't doing it already make sure she is backing up her photos. Amazon prime has free unlimited storage. I put all my photos on Amazon, computer hard drive, then a second external drive. External hhd drives are cheap.