Toph would be blunt about it. She might tell both Aang and Katara that they're being stupid if she thinks the break - up is over something silly. But if it's a more serious issue, she'd support them in her own way. She might offer a place for Katara to stay if she needs to get away from Aang, or she might give Aang some tough - love advice on how to handle the situation.
Katara could be heartbroken. She might initially be in denial, not believing that Aang has actually left. Then she could become angry, feeling betrayed by Aang's sudden departure.
One possible reason could be their different approaches to life post - war. Aang is the Avatar and has a whole world to look after, while Katara has her own tribe and personal goals. Their busy schedules might pull them apart. For example, Aang might be constantly traveling to different nations for diplomatic and Avatar - related duties, leaving Katara feeling lonely. Also, they might have different ideas on how to handle future threats. Aang may prefer a more pacifist approach which Katara might not always agree with, especially if it endangers the people she loves.
Sokka would probably be overprotective. He's always been protective of Katara, and now with her being pregnant, he would be constantly worried about her safety and the well - being of the baby. He might offer all kinds of advice, some of which might be a bit silly but well - intentioned.
Aang might go into a deep state of depression. He has always been an emotional character, and having his heart broken by Katara, someone he loves deeply, could send him into a downward spiral. He might isolate himself from his friends and stop practicing his bending for a while.
Some of their friends from the Fire Nation, like Mai, could be hesitant. She might worry about the implications for the Fire Nation's future and Zuko's role as a leader. However, as time passes and she sees how much Zuko loves Katara and their unborn child, she becomes more accepting. On the other hand, the Water Tribe elders could have mixed reactions. Some might see it as a good sign of unity, while others might be concerned about cultural differences.
Evie would probably be very sad. She'd try to be there for Mal, listening to her side of the story and offering advice. She might also be a bit disappointed in Ben if she thought he did something wrong.
The Fire Nation, which has now become more of an ally, would also see this as a positive thing. Zuko, being Aang's friend, would be happy for them. He would see it as a sign of peace and unity among the different nations. The Fire Nation people might send some precious gifts as a sign of their respect for the union, as they know how important the Avatar and Katara are to the new world order.
Some of their other friends from Gryffindor might take sides. There could be a group that blames Harry for being too preoccupied with post - war matters and not giving enough attention to Ginny. Another group might think Ginny is being too impatient with Harry's recovery process. This could create some tension among their friend group for a while until they all learn to accept the break - up.
Sokka would be protective of his sister. He would probably start by trying to negotiate with Zuko's side to call off the marriage. If that doesn't work, he might come up with a plan to rescue Katara, perhaps enlisting the help of their other friends and allies.
Their relationship could deepen as they share in the joys and sorrows of rebuilding the world. They've been through a lot during the war, and now they can truly rely on each other. They might become even closer friends at first, with a mutual respect for each other's abilities growing stronger.