Edward's reaction could be complex. He might initially try to respect her decision and give her space. But as days go by, his longing for her would grow. He could start writing letters to her, pouring out his feelings, but never send them. In his vampire family, he would become withdrawn, not participating in their normal activities. He might also have dreams about Bella, which would only make his pain worse. Eventually, he could try to change himself, thinking that if he became a better vampire or a more understanding partner, she might come back to him.
Well, at first, Edward would be in shock. He wouldn't believe that Bella actually left. Then, he would likely start to search for her frantically. He might go to all the places they used to go together, hoping to find her. His reaction could also be one of self - blame. He would think that it was all his fault, that he didn't protect her well enough or love her the right way. As time passes, he could become more resigned to the situation. But he would never really forget her. He might even try to keep an eye on her from a distance, just to make sure she's okay, even though she left him.
Edward might be initially shocked. He has always seen Bella as his own and this sudden change could be hard for him to accept. He could become withdrawn and moody, similar to how he was before Bella entered his life.
Edward might be initially shocked and heartbroken. He has been in love with Bella for a long time, so her leaving for Jasper would be a huge blow. He could become withdrawn and moody, brooding over what went wrong.
She could be heartbroken. After all, she's pregnant and the person she loves has left her. She might feel abandoned and alone, crying a lot and wondering why Edward would do such a thing.
Often, Bella's reaction is one of shock and disbelief at first. She might not understand how Edward could leave her for Tanya. Then, she could either isolate herself from the vampire world completely or she might seek revenge in some extreme fanfiction scenarios. She may also start to question her own self - worth, thinking that she wasn't good enough for Edward.
She might be completely devastated. Bella has always been deeply in love with Edward, so this sudden abandonment could shatter her world. She could be left in a state of shock, unable to really process what has just happened.
He could be devastated. Edward is deeply in love with Bella, so he might go into a state of depression. He may isolate himself from his vampire family and friends, constantly replaying in his mind what went wrong in their relationship.
It could be that there are external forces at play. Maybe there are other characters or some kind of dark secret in the fanfiction lore that forces Edward to leave Bella while she's pregnant. For example, there could be a threat from a rival group that says they'll harm Bella and the baby if Edward stays, and he thinks leaving will draw the danger away from them.
Fem Harry might be heartbroken. She could initially try to win Edward back, using her magical abilities in creative ways. For example, she might create a magical memory of their time together to show Edward what they had was special.
Together, they would likely try to make Renesmee understand that their relationship is special and that her presence is also special. They might give her a hug or some form of affectionate gesture to show that they are a family unit and that her walking in is just a normal part of family life, even if it was an intrusion on their private time.
Another reason could be the influence of other characters. There might be a new character in the fanfiction who convinces Bella that Edward is not who she thought he was. This new character could plant seeds of doubt in Bella's mind about Edward's intentions and actions, ultimately resulting in her hatred.