The police tend to view groups of youngsters meeting up late at night as potential trouble - because they often are.
If your friend is being targeted under this suspicion, then the simple ansswer is to meet socially elsehere - a restaurant or a pub, or as suggsted, someone;s house.
Swearing is a Section Five offence, and the police don;t apreciate it - and your friend got the the warning which is usually offered.
Your friend may be within her legal rights, but does she really need the grief? I'm sure the police don't.