Octavius could have mentioned that Lavender had brought up the idea first. He could have but he knew what she was doing. It wasn't uncommon for someone to be allowed to bring a plus one to a wedding. She had to know that. He would not have asked if he had been told that he were not allowed to bring someone.
She wanted a way out of it. He was compelled to go easy on her . This was all new and perhaps even a bit scary. Really, he was happy that she hadn't thrown it back into his face. It was an awfully couply thing to suggest.