A possible glitch worth reporting. One of my students is importing a graphic from Illustrator into XD and the gradient is reversing (switching the colors from left to right). When adding it to the Assets library and applying it to a shape in XD, it flips around top to bottom. It's easy enough to change the orientation but reversing the colour order is a little more work.
We are sorry to hear your student having trouble importing gradients. Would you mind telling us the XD and the OS version of his machine? Is he is facing issues with all files or with this specific file? A screenshot or a video recording can help us for better understanding.