To be pregnant, 2 criteria should be fulfilled:
1. Fusion of sperm with ovum to form a fertilized ovum.
of the fertilized ovum in the Uterus
So, the involvement of sperm is needed.
The pre cum (pre ejaculatory fluid) also has low levels of sperm.
So, even if this pre ejaculatory fluid (pre cum) enters into the genital tract, the sperms present in it can fertilize the ovum.
Hence, resulting in pregnancy
methods available that can prevent pregnancy are the emergency pills(like ipill) which provide protection only if taken within 72 hours of unprotected intercourse
However, as these pills contain high dose of hormones, they interfere with the regular cycles & cause delay in the subsequent cycle.
There is no need to worry, your friend can wait till she gets her periods.
If they are delayed, a Pregnancy test can be done to confirm it.
In case of pregnancy, it will give a positive result within one week.
Wishing you a Good Health.
Let me know if I need to clarify further.
Thanks & take care.