Removing/fixing a background with a gradient on an JPEG image.

Hi everyone! I hope you all are being safe amidst this pandemic 🙂

I decided to make the most of my quarantine and wanted to spice up the walls of my room, so I did. I found a wonderful artist who I commissioned to draw a digital portrait of my cat, as one always does, so that I later could send the picture off to a printer to get it professionally printed with good quality and then frame and put it up on my wall. I received the finished picture (as seen down below) and it turned out way better than I could ever imagine but after looking at the picture a while I started seeing some weird pixelations in the background. I send her a screenshot and she replies with "that's what happens when I export it as a Jpeg image" but the portrait is scarp and gorgeous so it's the gradient function in the program she's using that might not be the best. Fair enough I said and asked her if she could send me another version of the picture but on this one, she removes the "background" layer and exports it as a PNG (so I can make my own gradient to apply it on, or maybe just add it on a white or black surface). I even tried printing the picture at the art school on some ordinary paper to see if these malformations would show up and they did so the original picture is a no go if I don't fix the problems. She is very sluggish with it, taking her time doing these small adjustments and it looks like she will not even try because the commission is delivered.

So that is why I am here today, is there any way of fixing the gradient without ruining the portrait? The picture came as a JPEG file, 3500px x 3500px with 200dpi. I am not the best at photoshop but with some directions, I think I could handle it! Your help is much appreciated!

Lots of Love,  NewbieArtStudent

*edit* The print I am going for is only 20x20 inches (or 50x50cm) so that should not be a problem.