There is no "threaten to sue" there is only sue or not sue. Until you are served with a court summons and complaint you are free to ignore the threats and go on your merry way.
However, you do have a contract, an oral contract, which is enforceable, especially since you did live there for a time.
So, yes, you can be sued for the breach of the contract.
You might be able to defend against part of that $2000.
You were basically a subtenant of your roommate's father if his father signed or co-signed the lease. Or your roommate's subtenant if your roommate was the one who signed the lease.
The problem is this.
If you have the money and are just disputing the amount, then offer a lump sum cash settlement and get the settlement agreement in writing before you hand over any money.
However, if you don't want to pay because you don't have money, then talking about paying without having any cash to back up your mouth is likely to get you sued and whatever they win is a judgment that goes on your credit report and allows wage garnishment if you still can't come up with money.
So, which is it?
Do you have money, or don't you?

