Boo Radley mended Jem’s pants.  There are several reasons we know this.  First, we know that Boo Radley was aware of the children’s presence. As Boo was always peeking through the window at the children, we can assume he watched them run away.  As they run away, Jem loses his pants “Dill and I rolled through and were halfway to the shelter of the schoolyard’s solitary oak when we sensed that Jem was not with us. We ran back and found him struggling in the fence, kicking his pants off to get loose. He ran to the oak tree in his shorts” (Chapter 6).  Jem later returns for his pants, and he later tells Scout, ““When I went back for my breeches—they were all in a tangle when I was gettin‘ out of ’em, I couldn’t get ‘em loose. When I went back—” Jem took a deep breath. “When I went back, they were folded across the fence… like they were expectin’ me” (Chapter 7).  Jem’s pants were torn and tangled, but when he returns for them, they are folded neatly over the fence, and they are mended.  The reader knows who mended them because Jem says, “They’d been sewed up. Not like a lady sewed ‘em, like somethin’ I’d try to do. All crooked” (Chapter 7).  No one other than Scout, Dill, and Jem knew about the pants.  Mr. Radley did not know, or the children would have been in trouble.  The only other person there was Boo Radley.  Given his interest in the children and the presents he left for them, the crooked stitches and the neatly folded pants reveal to Jem and Scout that Boo mended the pants.