Can legal action be taken in this situation?

My mother has been working at a small shop for seven years now.

She suffers from fibromyalgia, a condition which causes muscular pain and exhaustion and so her working hours are limited. One of my mother's colleagues is a heavy drinker and she is constantly phoning in sick and turning up drunk for work. When she phones in sick, my mother is asked to cover, despite her illness. After a year or so of this, my mother finally spoke out about the fact that her "sick" colleague is clearly taking days off work because she is hungover or wanting to stay home drinking. My mothers boss is completely ignorant to this blatantly obvious fact and the colleague in question denies it profusely and constructs elaborate lies about the severity of her illnesses. The woman has since confessed to lying about her illnesses in a series of threatening texts to my mother and the boss is still being completely ignorant and has called my mother a liar on several occasions, and also stated that "she is lucky to have a job as she is disabled and no one else would employ her" (although my mother is technically disabled, she receives no money from the government). Other colleagues who are aware of the goings-on have seen how my mother has been treated and are now denying any knowledge of the alcohol problems of their work colleague and also of the inappropriate comments of the boss towards my mother about being lucky to have a job as they are scared of losing their jobs in the tough economic climate. Due to this, when my mother cites them in her arguments and they lie and claim to know nothing, it only reduces my mothers credibility.

Can legal action be taken?

I am in the UK by the way.

Thanks in advance.