It may be easiest to be the bridge between the two, anonymous. What I mean is try befriending the riolu yourself first. Coax him into trusting you (preferably without simply giving it treats, given that one should never feed wild pokémon) and build a relationship with him. The more your zorua will see you interacting with this riolu, the more she’ll be comfortable befriending him herself, partly because you would be showing her that it’s okay to befriend this riolu and partly because you would be showing her methods to approach him calmly.
If that doesn’t work (that is, if your zorua is still too shy for interaction), you may need to work on building her confidence. If you have any other pokémon or know any other trainers, have her practice interacting with others. Feed her treats for positive interactions and be as supportive and encouraging during these play dates as possible. It might even help to have her spar with others in order to help her feel comfortable with herself.
Then, when you encounter the riolu again, stand by her side and keep her calm as she approaches. Have her approach slowly but with purpose, and have her begin with calmer actions (such as sniffing riolu as a hello). Perhaps allow her to offer a toy or a treat (yes, even though humans shouldn’t feed wild pokémon). Ultimately, let the riolu lead the interaction, but be present to help your zorua stay calm and in control.
Incidentally, some blogs may suggest that zorua disguise herself as a riolu, but this is actually not the best idea. It’s far better to approach a new friend as you are so you can feel comfortable being yourself around them later. Having your zorua disguise herself as something she’s not could lead to confusion, disappointment, or even hostility down the line if she drops the illusion in front of her new friend.
Best of luck, anonymous!