If someone has unprotected sex during an outbreak, didn’t realize tell the next day, then what are the chances approx increased to that the virus will be transmitted? 

We don't have a number to fit that answer I"m afraid, but I would say that the risk of transmission in that situation is relatively high.  The susceptible person should observe carefully for lesions in the next 2-10 days and if one appears, they should have it swab tested for HSV---