@shewy92 We're not shown why they broke up, and Gyaru seems to be one to do things on whims. They have an okay relationship post breakup, he gets along well with her friends and she gets along well with his family, and he seems like an earnest and okay guy. After all, in this chapter he interprets Dinosaur as telling him that she's had a ton of guys over, and he doesn't come to accuse her, just to say he wants to get back together.
Some people break up and get back together and it all works out in the end. Who knows what caused it? Whatever it was wasn't even traumatic enough for her to redecorate her room, at least.