Danny would probably be heartbroken. He really cares about Sam, so finding out she cheated would be a huge blow. He might go into a state of denial at first, not wanting to believe it.
Danny might react with panic at first. After all, he has been hiding his identity from his parents for so long. He could try to escape from them, using his ghost powers in a desperate attempt to get away before they can do anything drastic.
Danny would probably be extremely hurt. He might initially go into denial, not wanting to believe that his friends could do such a thing to him. He'd keep trying to reach out to them, hoping it was all a misunderstanding.
The Joker would probably be both intrigued and excited. He might see Danny's powers as a new and interesting toy to play with. He could try to find ways to manipulate Danny into using his powers for the Joker's own chaotic purposes. For example, he might trick Danny into using his intangibility to break into a high - security building so the Joker can plant some crazy bombs or something.
Their friends might be really shocked at first. Tucker, for example, might not know how to handle the situation. He could be worried about how it will affect their group dynamic. Jazz might be more understanding and try to offer support, but also be a bit concerned about the practicalities of them being so young. Some of their other friends might be judgmental, thinking they're too young and not ready for this big step.
In this fanfiction, it could be that Danny Phantom meets a Danny Phantom from a parallel fan - made story. At first, they don't trust each other. But as they learn more about each other's worlds, they realize that there are some common elements that are being threatened. It could be that a powerful ghost is trying to rewrite all the fanfic universes related to Danny Phantom. So, they must combine their strengths and knowledge to stop this from happening.
Sam would probably be really intrigued. She might see it as a cool new form for Danny and would be eager to learn more about how it affects his powers. Tucker, on the other hand, would likely be more concerned about how to reverse it at first. But once he realizes that Danny can still do some cool things as a mermaid, he'd be excited too.
Well, in this kind of fanfiction, Danny could be controlled by an ectoplasmic parasite. The parasite latches onto Danny and takes over his actions. His eyes might glow a different color to show he's not in control. His friends would first be confused, then realize something is amiss and start investigating.
There could be a fanfic where Danny and Ember accidentally swap powers. This leads to a series of comical and also dangerous situations. Danny has to figure out how to control Ember's power of influencing people through music while Ember struggles with Danny's more combat - oriented powers. It could be a story full of self - discovery for both of them.
Another possibility is that he's facing some emotional turmoil. Danny has a lot on his plate. He has to keep his identity a secret from his friends and family, deal with school, and fight ghosts. All this stress could be causing him to lose his appetite. He might be too worried or preoccupied to even think about food.