Trahearne responded quite readily to Alastor's message:

There will be fires and a grill for you to use, so there are facilities available for cooking depending on what you plan on bringing. Come any time in the afternoon and stay as long as you like.

So he honestly was not expecting for Alastor to rock up with a meal to feed the entire party several times over. His eyes widened in shock when the sight greeted him.

"Alastor?" There was disbelief in his voice as he strides over to offer a hand to carry something. "Did you make all of this?"