Welcome to HCM,
Your grand mother is hypertensive and taking medicine for it usually once a day medicine is given to have better compliance and such drug are long acting or sustained release formulation.
You have not mentioned what drug she is taking?
Please visit your Physician he can split the dose at 12 hourly interval, or he/she can reduce the dose and add another drug in the evening.
Please do not worry it is possible and very simple so see your physician, he/she will certainly help.
Some time there are anti anxiety
drugs which are given hence you should not split the drug by your self.