Perhaps her parents dun wan their daughter to get into trouble.
It would be heartbreaking for her parents to realise that somthing
real bad happen to their daughter. Worse, she would be traumatise
by the incident.
Who is to say that nothing can happen during a party, after several
rounds of drinking, no matter how decent that group may seem to be.
There will always be a devil around
The night can be very tempting to some guys 
I think, if her bf has any respect for her gf, he would understand
the situation. If not, then, good luck to her after her marriage
with him.