In what way is the policeman impressive?

The policeman, Jimmy Wells, is impressive in two ways: as a friend and as a professional. As a friend, he shows just how much his friendship to Bob means by turning up at the exact time at the exact place at which they'd arranged to meet all those years before. Although Bob is a thief and Jimmy is a cop, Jimmy still clearly values his friendship after all these years. That explains why he doesn't want to subject Bob to the indignity of being arrested in person by his old pal. He leaves that to a detective.
On a professional level, Jimmy is impressive for the way he goes about his work. Despite his long-standing friendship with Bob, Jimmy is still a cop, and his overriding duty is to uphold the law. It would have been all too easy to let Bob slip away for old time's sake, but Jimmy does the right thing and helps to bring about his friend's arrest.