How can we tell someone the proper way for handling a dinner invitation?

Let me explain. For the past few years, my spouse & I are invited by the wife of a gentleman we know to celebrate his birthday. There are usually about 3 to 5 couples invited to attend a dinner at a fancy restaurant. Every time we receive this e-mail, "Margaret" says it's "Todd's Birthday." In the past, we brought a gift and had to pay for our portion of the total bill. How can one discreetly tell "Margaret" that she should NOT be saying that it is "Todd's Birthday" anymore. She tells us not to bring a gift & it's just an excuse to get together (at a fancy restaurant). You would think that she would know better because if she calls it a celebration for her hubby's birthday, then she should be treating all of us. Right? She never treats.