No I don't believe a person is responsible for what someone else writes. Sounds like she would be judgment proof. Your best bet is to separate yourself from doing business with these types of persons - business not family - is what I'm referring to...
No liability for you or your friend. People write bad reviews all the time, it is part of doing business. Your mother is free to express her opinion; however, she cannot make false statements of fact which would harm another's reputation - that could be construed as defamation.
I am not familiar with the circumstances underlying this particular incident. Please be advised that the foregoing is not legal advice and in no way constitutes the formation of an attorney-client relationship. Do not take action based on the above. Do consult with an attorney in your area as soon as possible. If you are in New York State contact me directly at (607) 936-8057.
I suppose my question is what is the basis of your mother's review? Assuming it is things you told her, then it may be possible, if there are things that are untrue, that they could trail back to you or whomever provided her with the information. In that instance she would simply be publishing, or republishing the untruths.
No information provided in response to these questions can be relied upon in any way without further personally consulting with Attorney Kerrigan and Attorney Kerrigan consulting personally with you regarding your specific legal situation.