Singer Heavy Duty. Best bang for your buck, easily. On Amazon. $120.
https://www.amazon.com/gp/product/B003VWXZQ0/ref=oh_aui_detailpage_o06_s00?ie=UTF8&psc=1
My wife uses this sewing machines almost daily - https://www.amazon.com/SINGER-4423-Heavy-Sewing-Machine/dp/B003VWXZQ0/ref=sr_1_3?ie=UTF8&qid=1504045144&sr=8-3&keywords=singer+sewing+machine
It's cheap, get's the job done, and great for learning on. It can sew though (most) denim with no problems.
-Bahzad
If you have any say in the model, ask her to consider the Singer 4423 HD. It’s $199, fully mechanical (no computer or electronics) and built like a freaking tank. The frame inside is completely metal. Bobbin loads by dropping it in under the foot. Works with both normal spools and the long skinny spools that lay down. Weighs over 12 pounds.
I’ve used mine to sew thick patches on my kids’ karate uniforms and go through multiple layers of “pleather” for cosplay costumes. Nothing stops it.