My girlfriend got a Postinor and on opening it one of the tablet broke into half. So she took just 1and the half tablets. would it still be effective? what should she do?

Hello and welcome to EmpowHer,
From what I understand she took both halves of the pill. If this is correct, then it may still be effective. However, neither I nor anyone else can tell you the exact effect it will have on her body. If she experiences the normal side effects of the pill (spotting, cramping, tender breasts) then its possible the drug worked.
At this point she needs to wait for her next period. If she misses her next period she can take a pregnancy test.
Faith