How is Widge harmed by Adam?

Adam steals Widge's savings. When the boys arrive at Leicester to stay at Simon Bass's, there are already people living and working there. One of them is Adam, who works in the stables. Adam is a large boy who Widge feels will have more privileges than him because of his tenure there. When Adam steals the money Widge brought with him, Widge decides not to turn him in, because he doesn't think that anyone will believe him over Adam.
Since Adam already understands the city and the household, he's able to take advantage of the newcomers. They won't be given the benefit of the doubt.
Adam's theft of Widge's money harms Widge because he doesn't have a lot of ways to replace it. He's only there to steal the play; he doesn't have the time or the opportunity to replace what was taken without difficulty.