Work with your boyfriend's criminal defense attorney to help uncover the truth.
In no way am I offering you legal advice, and in no way has my comment created an attorney-client relationship. You are not to rely upon my note above in any way, but insted need to sit down with counsel and share all relevant facts before receiving fully-informed legal advice. If you want to be completely sure of your rights, you must sit down with an experienced criminal defense attorney to be fully aware of your rights.
If you were a witness, you need to let his defense attorney know. If you weren't, your opinion isn't relevant.
Macy Jaggers's answer to a legal question on Avvo does not establish an attorney-client relationship. Ms. Jaggers offers everyone a free consultation to discuss their case. Feel free to call her office at 214-365-9800 to make an appointment (phones are answered 24 hours) or visit her website at www.macyjaggers.com for more information about her services and recent victories.
As my colleagues have stated, you should work closely with your boyfriend's criminal defense attorney. If he has not yet retained a criminal defense attorney, then you should make that your top priority. The prosecutor will likely take this case very seriously. From what you said it sounds like a class A misdemeanor - which carries a maximum punishment of one year in county jail and/or a $4,000 fine. However, depending on the nature of the offense and your grandmother's age, the State may elect to charge your boyfriend with Injury to an Elderly Individual. It is impossible to say which degree this charge will be without knowing more details about this case. I can say that if the State proceeds on this charge, your boyfriend will be looking at a felony of some degree. In any event, you need a competent attorney to advise you and help you through this. Good luck to you and your boyfriend.