First of all, if she is 18, the juvenile courts aren't involved. We use the term "minor in possession" to refer to anybody under 21 who is alleged to have possessed an alcoholic beverage, but she's in the twilight zone where she's considered an adult but still can't do everything an adult can do.
People give phony names all the time when they're arrested, and police officers do their best to make sure they have the right person. Usually, this involves calling dispatch to make sure the physical description and date of birth match the DMV records. If someone uses your name, it's a form of identity theft.
If she doesn't show up for court, as she promised to do when she signed the ticket, a warrant could issue for her arrest... and the next time she has an encounter with the cops, she may be going to jail in the back of a police car.
I would advise her to go to court even if somebody else had used her name. If someone is charged with a crime as the result of identity theft, the defense attorney would try to show the prosecutor they had the wrong person and get the charges dropped. However, it sounds like she was the person cited, so the cop would probably come to court and say, "Yes, that's her."
She's actually lucky they just wrote her a citation for possession of an open container. If she was convicted of Business & Professions Code 25662, minor in possession of an alcoholic beverage in a public place, she would lose her drivers license for a year (or, if she doesn't have one, she would have to wait a year to get a CDL).
You didn't say exactly which statute or ordinance was on her citation. There are local ordinances in many communities, such as Newport Beach, that make it a misdemeanor to possess an open container in a public place. That could result in jail (but it's unlikely). If you're charged with a misdemeanor and can't afford an attorney, you're entitled to a public defender. If it's an infraction, you can hire a lawyer at your own expense.