From what I understand, the BP was taken when the Dr. checked it after the accident and, accordingly, prescribed the medication. High blood pressure is a medical condition and, with relevance to the medical insurance policy, it is considered to be a pre-existing ailment. Although you mentioned he did not have health issues, please clarify - did your father ever have high BP prior to the accident? Has he ever undergone regular checkups during which this was discovered and hence he has been taking prescribed medication for it? If it was an already existing condition, then, yes, it is important for it to be disclosed to the insurance service provider and should have been mentioned at the time of taking the policy.
Now if the high BP showed after the accident, then it will be indicated in the Dr.'s note and there will be a written prescription of the medicines to take. In that case, you will be submitting the prescription to the service provider in any case to support a claim for the pharmacy bill for the amount paid. What you need to check is the policy's terms and conditions yourself, and not rely wholly on what the service provider tells you verbally. There are certain aspects you should be aware of, such as whether the insurance claim can be made only for hospitalization for a certain number of hours, pre- and post-hospitalization expenses, OPD charges (such as for a follow-up), etc. Find out what all is covered and to what extent (5%, 10%, etc).
The bottom line in my personal opinion is that you should be totally honest with the insurance company. If the BP itself is not really the issue, that is, that it may not be factored in as a cause of the accident, then I don't think they would reject the claim as such and you can clearly tell them that the high BP was detected during treatment for the accident and was not a pre-existing condition.
This is my personal opinion and you could consult some family friend or relative who may have knowledge about this.
One last thing to note- You also need to consider that you took the policy just three months ago and the accident happened within a few months of it. So it would be the first claim application that you are making in the year 2022-23. Hence, you would also need to find out if, God forbid, you are required to again apply for a claim again within the same period, would that be covered, too, and, if so, how much percentage of what is spent is covered by the policy.
When you make a commitment, you create hope. When you keep a commitment you create trust! ~ John C. Maxwell