Trying not to get pregnant. Best times for ?

my friends is tryin 2 have but she is just goin with the flow. she isnt not goin 2 have . but sum1 else told me that if as soon as u r off ur period, u can have the 1st 3 days n no chance of pregnancy and then the 4th day n week after is when u have best chances. is that tru? i dont want my friend pregnant cause her mom will kick her out.