Harassment for debt is a criminal offence but I can't see that the circumstances you describe would fall foul of that legislation. (An example of circumstances which might constitute 'harassment' occurred when a Sheffield newsagent filled his windows with signs saying things like "Mrs Martha Briggs of 43, Downton Crescent owes this shop £43.47". The police visited him to tell him that his actions might be breaking the law).
Further, I can't see that it's likely that the Data Protection Act was breached. It might have been if the caller said "Mrs Smith owes Capital One Bank £2745.68, which has been outstanding since March 2012" but simply indicating that "Mrs Smith owes someone some money" would probably not be deemed illegal as there's insufficient 'data' in the statement for such a ruling.
In general, anyone has the right to phone another person and enquire about a third party. (e.g. I could phone the Queen's secretary and ask what colour knickers the Queen wears, without breaking any laws). It's up to the person receiving the call to decide whether it's desirable, or legal, to disclose the information requested.
'Harassment' can't occur through a single action. There has to be repetition of relevant actions (e.g. if the debt collector kept phoning your friend's employer at frequent intervals) or continuous action (as with my example of the newsagent).
Further, if if there were grounds for a court case, I can't see any way that it could be publicly funded. Your friend would have to find the money to fund the case (which, presumably, she hasn't got) and run the risk of losing and having to pay the other side's costs as well.