Harold, slightly perturbed by the question, laughed awkwardly. "Well, you see, I have a vague sort of understanding of how one should dress for such an occasion; my father did attempt to educate me a number of years ago in etiquette and fashion - and while I believe I picked up some of the more... shall we say... more subtle fixtures of his teachings, dress sense was unfortunately not one of them. At any rate that would have been twenty years ago and I am given to understand that both wizard and muggle fashion sensibilities have changed since then."
He thought about it for a moment. "I am positive I can make a quite trip to Diagon Alley before the ball and pick up something appropriate from Twilfitt and Tattings - as this is presumably an event of such prestige that perhaps is beyond the otherwise-inestimable Madam Malkin?" Harold honestly thought that Madam Malkin was up to the task, even if she wasn't quite as long established as Twilfitt and Tattings, but he wanted to reassure Rose that he understood the general concept of what was expected here. "In any case, I'm certain they can demonstrate to me the latest fashion so that I shan't look out of place."
He added, with a raised eyebrow. "All the better to look 'undercover', I suppose." Harold internally realised that in the overall scheme of things, this ball would be slightly more of 'his' kind of people - people coming from money, people of claimed lineage. The kind of people that, quite frankly, he'd decided to somewhat leave behind.
He idly wondered if now was the time to rekindle the connection with his younger brother Edward who, as far as Harold knew, was still part of the family business and while not of the same strata of power, money or influence as the good folks he was likely to encounter at the ball, the Prendergasts were not historically unconnected.
He reflected, "You know, it is very kind of you to do this for me. I shan't imagine it will be the easiest of evenings - for either of us - being both on the prowl, as they say, for information... but I rather hope we can enjoy ourselves at least a little. I may not be the kind to, say, 'let my hair down' as such, but I'm sure we can find interesting people to talk to for the evening."
He paused, "Well, then, all that remains is to know when and where to be."