rude collector

my mother recently has had some collection agency call over and over. there is one woman in particular that is rude and threatens my mother with jail time. my mother got a $300 loan from a federal credit union in 2008, now the agency is saying she owes almost $1800 and needs to be paid within the next week or she will go to courct, possibly jail. my mother is on disability, has been for almost 12 yrs, and she cannot afford this by any means. we have no family members "with money" to help pay it so it will not get paid. when my mother tries to talk and work out some payment plans with this rude woman.. she starts yelling at my mother, threatening her, telling her to shut up. it is very unprofessional, how can this be legal?? can you offer any type of suggestions or opinions?