I think he's pretty clear on why they were there - as soon as Barty mentioned Selwyn's kids he completely put it together.
He has the opportunity to know that Ron has a friendly relationship with Arista that he maintains even though people like Finnegan give him guff about it; among other things Ron used the Stories About Satsuma to give him public posts to make during Umbridge, after all. And he knows that Selwyn considered himself a protector of the Weasley family.
Knowing these things and that Ron is a Gryffindor with perpelexing friendships with an assortment of other students - some of whom he may have specifically recognised - it only will take confirming that Ron sent a PM to Arista in discussions of the wedding to lock down "Arista managed to tell Ron that she was in trouble, and the little idiot got his friends together to break her out." Without the confirmation, he will only have "I bet Arista managed to tell Ron that she was in trouble, and the little idiot got his friends together to break her out."
no subject
He has the opportunity to know that Ron has a friendly relationship with Arista that he maintains even though people like Finnegan give him guff about it; among other things Ron used the Stories About Satsuma to give him public posts to make during Umbridge, after all. And he knows that Selwyn considered himself a protector of the Weasley family.
Knowing these things and that Ron is a Gryffindor with perpelexing friendships with an assortment of other students - some of whom he may have specifically recognised - it only will take confirming that Ron sent a PM to Arista in discussions of the wedding to lock down "Arista managed to tell Ron that she was in trouble, and the little idiot got his friends together to break her out." Without the confirmation, he will only have "I bet Arista managed to tell Ron that she was in trouble, and the little idiot got his friends together to break her out."