Does a collection company have to serve you if they want to sue you?

Or is there another sneaky way to get a judgment without you knowing it? My friend is paying these people every month and they are still not happy. Her husband lost his job and they keep threatening to sue even though they are being paid an agreed upon amount every month. She wants to know when they get a judgment can they go after her account without notifying her of a judment? She has not been served and only owes 1000 dollars. She has paid quite a bit on this bill and I dont know why they are still threatening her? It is making her nuts. She thinks there may be a way to file suit in another county that she lived in when the original bill was incurred? They know her current address, so is that legal?