Ron always felt vaguely uncomfortable in dress robes. There was something that just didn't settle right for him being wrapped up in something so formal. That's why he probably took the "semi" of "semi-formal" to heart. He was still in his more expensive robes but he had left behind some of the decorative details.
When he came to the beach he arrived with Mandy. He had asked her last minute to come, but then all the invitations had been last minute. He hoped this wasn't going to be too weird for her. Considering that this was supposed to be their first date and he had brought her to a celebration for his ex. Hopefully she'd have fun.
He felt pleased for Lavender, of course. She seemed to finally be getting her life on track. He was glad that she'd found some kind of happiness.