Why was my DMV hearing closed before the actual hearing?

It's hard to answer your question because DMV doesn't use the term "the hearing has been closed," so I'm not sure what you mean. When DMV dismisses a license suspension process, the usual term is "set aside."

This can happen for a number of reasons. When an officer arrests someone who chooses a blood draw, the blood isn't tested for a while and may come back under .08. When DMV gets the results, they set aside the suspension if it's under .08. (Note that a .01 standard applies if you're under 21 or have a restricted license due to a prior DUI.)

There are also police officers who don't know the law and file administrative paperwork when they believe a person is driving under the influence of drugs. The DMV administrative process is only for alcohol, so DMV sets aside the action when the review the paperwork and find the BAC is under .08.

The dismissal of the DMV action doesn't affect the criminal DUI charges, so she still has to deal with the court case. She can still be convicted of driving under the influence of alcohol, drugs or both, Vehicle Code 23152(a), if her blood alcohol was under .08. If DMV set aside the administrative suspension action, that might indicate there is a good reason to attack the criminal charges.

Please understand that this is a general discussion of legal principles by a California lawyer and does not create an attorney/client relationship. It's impossible to give detailed, accurate advice based on a few sentences on a website (and you shouldn't provide too much specific information about your legal matter on a public forum like this, anyway). You should always seek advice from an attorney licensed in your jurisdiction who can give you an informed opinion after reviewing all of the relevant information.