If they both received tickets, then both were cited. In California the citation should have the court date and location. It will be up to the courts to figure out the answers.
The matter is still under investigation. If it can be determined who was driving, then the appropriate charges will be filed, including, I presume, vehicular assault.
In any DUI case - the government must prove beyond a reasonable doubt that someone drove. If the government cannot prove who drove beyond a reasonable doubt, than neither your friend or you should be charged with DUI. If either of you is charged with a DUI, I would contact a local DUI attorney to represent you in this case.
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