Ask them what they mean.
Do they mean, for example, that people are still dying of diseases that couldn't be cured several decades ago? That's blatantly false - we have vaccines for more illnesses today than ever before, preventing deaths the world over. We have medications that can stop the progression of leprosy (Hansen's Disease) - a disease that would have caused a person in Bible times to have been ejected from their home and community and forced to live out their days destitute and disfigured.
Do they mean that there is more poverty in the world?
Do they mean that there is more unemployment?
Do they mean the world is more violent?
Each of these arguments has been proven to be false - the fact of the matter is that past centuries have been more bloody, far more violent, far more superstitious, far more impoverished, etc. New methods of warfare, for example, have made us cognizant of the potential for mass destruction, and far more engaged in peace making efforts.
The ability to save the lives of infants born prematurely, babies whose survival was unheard of just a few decades ago, has spared many families the pain and grief of losing an infant that our grandparents may have endured more than once.
The ability to control our reproductive capabilities in the last 50 years has allowed women to contribute to their families' livelihood - we are one of the most affluent societies to have ever existed. Couples can actually *plan* how many children they have, educate them, give them a standard of living that was unheard of a couple of generations ago. Women are no longer simply wives and mothers - they are appreciated for their intellect and their contributions in society.
Oh, and slavery was abolished like 150 years ago.
http://bigthink.com/the-evolution-of-enlightenment/the-world-is-getting-worse-and-other-lies