One option is to simply avoid reading or engaging with Rias fanfiction. If the OC hates it, there's no need to subject it to that content.
If the oc hates it but still wants to give it a chance, they could try to find a different take on the rias fanfic. Maybe look for ones that are more in line with their own values or interests. For example, if the oc is interested in action, find a rias fanfic that has more action elements rather than just the typical romance. Another thing to do is to try and write their own version of a rias - related story that they would like, incorporating the elements they enjoy and leaving out the ones they don't.
The OC can simply avoid reading any more of that fanfic. There are plenty of other stories out there in different fandoms or even within the 'High School DxD' fandom that might be more to their taste.
It could be that the OC creator simply doesn't like the typical tropes or storylines often found in Rias fanfiction. For example, if the fanfictions tend to be very formulaic or over - use certain clichés, the OC might be a symbol of a break from that. Also, perhaps the OC has a specific backstory or purpose that is at odds with the general narrative of Rias fanfiction.
Maybe the OC has a completely different personality or set of values from what is often portrayed in Rias fanfic. It could be that the OC has had bad experiences related to the typical tropes or themes in those fanfics.
Perhaps the fanfiction community could create some guidelines for writing Alola - related fanfictions. These guidelines could ensure that the stories are in line with Ash's character and the spirit of Alola. If the quality of the fanfictions improves, there's a chance that Ash might not hate them as much.
The writers could stop writing that type of fanfiction if they respect Ash's 'feelings' (assuming he were a real person with real opinions).
One thing that could be done is simply respect his opinion. Don't try to force him to read or like poker fanfiction. If others like it, they can enjoy it among themselves without involving Percy.
He could simply avoid reading it. Just stay away from any fanfiction platforms or books related to magic fanfiction.
If it really bothers her, she could try to find a community or group of people who also dislike Bella fanfiction and discuss her feelings with them. This way, she might feel less alone in her dislike and could even get some ideas on how to deal with it better. For example, they could share tips on how to filter out Bella fanfiction when browsing for new stories.
First, she can simply avoid reading it. There are so many other forms of Naruto - related content, like the original manga or anime, that she can focus on instead.