According to California Code of Civil Procedure section 1987(a), the service shall be made so as to allow the witness a "reasonable time" for preparation and travel to the place of attendance. I'd say tomorrow morning at 8:30 a.m. is not considered a reasonable time for preparation and travel for an elderly woman living in a nursing home.
If the trial is continued or your mother does end up going, be sure to ask for witness fees and coordinate with the subpoenaing party concerning the time of testimony so your mother is not waiting in the hallway all day.
But I agree, tomorrow morning is not reasonable.
Disclaimer: You should discuss your situation with a local attorney, as the above statement is a general overview and does not take into account all of the factual considerations in your case. Nothing in this statement creates an attorney-client relationship.