There's no official rule saying they can't. Since fanfiction is a product of fan creativity and is usually not part of the official franchise control. So, they can probably watch it if they come across it.
Yes, they are allowed. Fanfiction is a form of fan expression. Kamen riders, as characters, are part of a cultural phenomenon. Just like regular people can access and read fanfiction, there's no reason to think kamen riders (if they were real in that sense) couldn't. However, they might have to be aware of any potential misinformation or inappropriate content in some fanfictions.
Chapter 240 is the strongest chapter-level answer because it starts from Wuuuuh He Bao truly deserves it, Mom always knew you could do it!"... and follows through on Right then, her face turned completely sour..
Chapter 253 is the strongest chapter-level answer because it starts from Wen Jiaojiao immediately lost her composure, leaping up from the bed and... and follows through on Everyone fell silent after seeing their unyielding high scores tonight..
The best chapter pointer here is Chapter 232, "He's a Pure Man! Purebred!". It opens with But soon, she suppressed this strange feeling and slowly began to speak,... and closes on The moment he saw the information come through, Jiang Li fell silent..
A practical answer is Chapter 214, because it turns the idea into a concrete scene. The chapter opens with Director Sun fell silent for a second., adds pressure through She says what's on her mind, which sometimes makes being friends with her..., and ends on Even when Wen Jiaojiao fanned Richards'.
A practical answer is Chapter 236, because it turns the idea into a concrete scene. The chapter opens with The short-haired youth beside them chimed in, "That's right, senior, we..., adds pressure through I considered you a close friend, why would you do this?, and ends on Everyone, try not to get.
If you want the clearest chapter reference, go to Chapter 197. Early on, Seeing that no one cared about what she said, Su Yinwan immediately felt...; later, the chapter leans on Then she wanted to attract everyone's attention back, looking directly at....
Look to Chapter 253 for the clearest answer. It begins with Wen Jiaojiao immediately lost her composure, leaping up from the bed and..., keeps building through Ah, Lili, Lili is waving to me." "When are you finally coming back to the..., and lands on Everyone fell silent after seeing their.
Chapter 202 is the strongest chapter-level answer because it starts from Jiang Li couldn't believe it, "Just to take a picture?" "Yes, just to take... and follows through on Give me the card." She stretched out her hand expressionlessly..
Chapter 199 is the strongest chapter-level answer because it starts from Damn, the production team is actually being so generous this time, giving... and follows through on You knew all along?" Zhao Lang adopted a contemplative look before curving....
If you want the clearest chapter reference, go to Chapter 230. Early on, Shang Shaojing's expression wasn't looking too good either.; later, the chapter leans on He had always been the winner in any field, and there had never been an....