In the BSD manga, unfortunately, Dazai did die. His death was a crucial moment that influenced the direction of the story and the relationships among other characters.
The Beast would be very confused at first. He has been the one with the beastly form for so long, and now seeing Belle in that state, he might feel a sense of guilt as if it was his fault. He would be protective of her, though, and try to help her adjust. Gaston, on the other hand, would probably see this as an opportunity to prove his 'heroism' by trying to 'save' Belle from her new form, but really he just wants to show off. Belle's father would be heartbroken and try to find a cure for his daughter, using his knowledge of inventions and books.
Their friends would likely be a mix of shocked and excited. Raven, being the more reserved one, might initially be a bit concerned about how this will change their dynamic as a team. But she would also offer her support in her own quiet way. Cyborg would probably be the most excited, already thinking about building all sorts of baby - related gadgets. Starfire would be overjoyed and would immediately start planning baby showers and knitting baby clothes. Robin would be more practical, thinking about security measures to protect Terra during the pregnancy and the new family once the baby arrives.