Ford F-150 - 1997-2003 :: 1999 - Died While Driving - Won't Start?

My niece has a 1999 F150 with a 4.6L engine. She was driving it down the road and it died and would not start. She called her husband, he went where she was, and when he got there, it started. She drove it home, and the next day, same thing. Died going down the road, no start, let it sit overnight, cranked like a new truck. Now it has died and will not start at all. They replaced the fuel pump. Nothing. When you turn on the key, the fuel pump runs continuously. If you pour fuel in the air filter, it will start. They checked all the fuses, nothing blown. When they try to hook it up to an OBD scanner, it does not work. No codes, nothing, it won;t pick it up.