I have found my blood pressure is high for a period of time (weeks) for no apparent reason. Then it goes the other way down below normal. There is no identifiable cause that I know about.
I take Lisinopril (ACE inhibitor) twice a day as well as Coreg (beta blocker). You indicate your mother is taking medication once a day? There is a doctor that gives advice wseekly on TV, and he stated he has seen better results when the medication is taken at night before boing to bed. He stated he did not know the reason, but the results were evident.
I hope they didnt simply take her blood pressure and say "hmm this is high, lets prescribe some medication". Have they actually done any tests to see if they can establish WHY its getting so high? Have they done blood tests to look at kidney function for example?
It sounds like her morning dose is wearing off in the evening. By giving her the additional dose in the evening this will keep her BP down. Her doctor is correct with doing this, but if her BP doesn't improve then her doctor needs to know this.