That both "Silky" Bob and Jimmy Wells have duly turned up for their appointment as planned says a lot about the deep friendship they once shared. For most people, the idea of meeting up with a friend twenty years from now in the exact same place as the last time meeting is too strange for words. Even close friends can drift apart over such a long time. Few people would demonstrate the same commitment to making such a solemn promise, never mind keeping it.
But Bob and Jimmy are different. Bob may be a serial criminal, but when it comes to his old school buddy he always keeps his promises. As for Jimmy, one gets the impression that he would gladly have turned up for the big meeting even if he hadn't been a cop setting up Bob to be arrested. Whatever line of work he was in, he wouldn't have missed this appointment for anything.
Bob and Jimmy have taken radically different paths in life, but they are still bound by ties of friendship. That explains why Jimmy can't bring himself to arrest his old pal; a detective does it instead. It also explains why we can be so sure that Jimmy would've turned up to his appointment with Bob after all this time, even if he hadn't been a member of New York's finest.
Further Reading