His friends would be overjoyed. They've witnessed Ash's growth over the years. For Brock, he might see it as a validation of Ash's skills that he's made it to such an elite group. Misty would probably be bragging about her friend to everyone she meets. And his other friends from different regions would be eager to visit him at the Elite Four headquarters. They would also be curious to see how Ash would interact with the other members of the Elite Four, and would be supportive of him as he takes on this new role in the Pokémon world.
His friends would likely have a mix of reactions. Some, like Pikachu, would be right by his side and excited for the new adventure. Others, like his human friends, might be a bit sad at first because he'd be spending more time with the Elite Four duties. But overall, they'd be proud of him. They know how hard he's worked to get to this point in his Pokémon training journey. And they'd also be looking forward to seeing how he'd change the Elite Four with his unique style.