Ron was still a bit worried that his brother and sister were going to overindulge Percy's senses with their "intervention." Ron understood that they were well intentioned, but after seeing him when he told him about Penelope's death, it was hard for Ron to consider how he would react to this. Or if it was truly necessary now that Ron had got his way with the watch on Percy thanks to Minister Kingsley.
So while Ron would rather have not come at all, he felt he needed to come and make certain that his brother and sister did not overdo whatever it was they had planned. Especially Ginny, because she had a tendency to get a bit - rowdy. When he entered the shop it looked as if Ginny had just walked in and he nodded to her and then to his brothers. "Hey everyone. Is there a party I didn't know about? Had to ask George a question."