Depends

Sounds like you are only a witness to the pending suit. You have no obligation to cooperate with them except that it is a subpoena and you must do something about it. The thing you say in the deposition may come back to hunt you later on. You never know. Take a look whether the subpoena is filed with the court.

Regardless the situation, I would say make them work. You could file a motion to quash or the employer could file a motion to quash. Even if you decide to cooperate with the subpoena, at least ask the law firm for witness fees. Check your local rules what under the law you can and cannot do re the subpoena. Try to get an attorney to go with you for the deposition as well.