Quitting her job is an option, but I think she should keep on with the complaints. If her boss doesn't like it, tough titty. Systems have reasons for existing. She has a right to file any number of complaints she wants, as long as there are an equal (if not greater than) number of reasons for her complaints. This shit shouldn't be allowed to happen, but if she doesn't complain and lets the assholes trample all over her, then the people are getting away with it, and nothing changes.
I don't know of any organizations that could help, especially in the UK, but I'm sure they exist. In this day and age, people with similar difficulties band together a lot, and there tend to be a lot of governmental agencies that strive for human rights for all people, not just "normal" people.
Some quick Googling found this: http://www.direct.gov.uk/en/Dl1/Directories/DG_10011138
"The Equality and Human Rights Commission is a non-departmental public body which works to eliminate discrimination, reduce inequality, protect human rights and to build good relations. It has responsibility for the roles previously undertaken by the Equal Opportunities Commission, the Commission for Racial Equality, and the Disability Rights Commission."
I'm not certain, but it may be helpful to contact them and tell them about the situation. It seems like there is a focus on discrimination between employees and employers, but I'm sure it's not that uncommon for customers to harass employees with disabilities, so hopefully someone will be able to help or at least give some advice.