Find out their address by doing a whois lookup, then write them a letter explaining the situation and ask them to remove them. If that doesn't work you'll need a lawyer.
She probably cannot be held accountable for his suicide. However, the possible theft should be immediately reported to the police department. They will be able to investigate and see if there is a criminal charge. Just from what you have written, there may not be, as he gave her access to the accounts. However, until someone investigates further, you just don't know.
As far as being able to recover the money from her, if there is a criminal case, restitution will be ordered by a court if she receives probation. If no criminal charges are filed, then you can get a lawyer and sue her. You still will have the same problem (that your grandfather gave her access). If he was obviously mentally deteriorating to the point where it was obvious that he wasn't "knowingly" giving consent, that will play a role in any civil or criminal proceedings as well.
Good luck. You might want to see if a civil attorney will give you a free consultation, but I would report to the police before spending the money to hire a lawyer.