If my girlfriend's soon-to-be ex-husband's lawyer sent me a subpoena for a deposition at his lawyer's office, do I have to answer it?

Assuming that the subpoena was properly issued, it has the effect of a court order.  If you fail to show up, you are subject to arrest.  If the time is incovenient, you should be able to reschedule.  However, you can't avoid appearing without permission from the lawyer who issued the subpoena.