Help for a friend

My BFF had unprotected sex on 3/1 it was her last day being fertile. She had sticky discharge and all that good stuff. She tested on the 14th it was negative. Has been having cramps like AF was coming but she is now 5 days late and another BFN can someone explain why. ..I thought it was too soon for a positive to show. ..can someone explain a time line of events and if they think she may actually be pregnant. Thanks : )