Thank you for your questions. You had several, so please allow me to address of the general legal principles in the order that they presented.
I should start by saying that because the nuances of every situation are different, this information should not be construed as complete or advice without consulting in person with counsel. For example, I can say with certainty that there may be additional legal theories to defend your case, for example, but there is simply no way to intelligently discuss those theories without consulting in person and being able to view the documents.
That said, You described a situation where a woman is a "customer", but also as "the contact person" for the main person on the account. Whether or not the woman is actually a customer is important. A service is being provided, and the contracting party is going to be liable for those services received, and anyone else is typically not going to be liable for the debt unless they are also a party to the service agreement. So if you have two adults living in a house together, and one of those adults is your customer and the other just the boyfriend/girlfriend "contact" of the customer, the "contact" is normally not going to be responsible for the bill because the contact will generally not be considered a contracting party.
So that said, if both parties are contracting parties, there would typically be no case. That may answer your question right there.
If you send someone into collections who is not a customer, then there would typically be liability because sharing a home with a debtor or being their contact person for a debtor does not make them responsible for a debt. Any foreseeable damages caused by sending such a person into collections would generally be the responsibility of the business that was responsible.
However, even then, it's not that simple for the plaintiff. Even if the defendant wrongfully sends the plaintiff into collections, and even if foreseeable damages result, the plaintiff is responsible for mitigating the damages. In the case of an action that adversely affects one's credit score, this means doing things like providing documentation of payment if payment was made, or challenging the debt with the three major credit bureaus. In other words, a person can't just sit back and let the damages accumulate, then sue for the damages when something could have been done to minimize or eliminate them.
Finally, the burden of proof
falls on the plaintiff. This means that the plaintiff must show that any damages are, in fact, the foreseeable result of negligent act. Typically, a 4 year old bill sent to collections will still appear on someone's credit score, but it will only impact the credit score by a few points. Depending on the dollar amount, it might typically lower a person's credit score by only 10-30 points. 10 points would typically be insignificant. 30 points is more likely to be significant, but if it lowers the plaintiff's credit score from 600 to 570, it really wouldn't matter either way because no bank in the country would give her a home loan
with a score of 600. So when someone is suing for damages due to bad credit, they need to demonstrate what the actual effect was on their credit score, and that it made a difference.
Your final question was whether you should get an attorney to represent you. Over $5,000, I would definitely recommend that you at least consult in person with an attorney to help you draft a response and examine all possible legal theories. Whether it is economical to hire that attorney for full representation
is a determination that you can make after you have had a chance to meet in person. It's entirely possible that you will decide that you can handle the matter on your own or that it doesn't make economic sense (e.g. it might cost you $3,000 to hire the attorney, and the attorney thinks she would actually be entitled to $2,000).