Or you could tell them about the STEP project where the best performing group was the one not prayed for.
The STEP project[edit]
Harvard professor Herbert Benson performed a "Study of the Therapeutic Effects of Intercessory Prayer (STEP)" in 2006.[41] The STEP, commonly called the "Templeton Foundation prayer study" or "Great Prayer Experiment", used 1,802 coronary artery bypass surgery patients at six hospitals. Using double-blind protocols, patients were randomized into three groups, individual prayer receptiveness was not measured. The members of the experimental and control Groups 1 and 2 were informed they might or might not receive prayers, and only Group 1 received prayers. Group 3, which served as a test for possible psychosomatic effects, was informed they would receive prayers and subsequently did. Unlike some other studies, STEP attempted to standardize the prayer method. Only first names and last initial for patients were provided and no photographs were supplied. Catholics and members of the New Thought church Silent Unity who prayed for the patients "were allowed to pray in their own manner, but they were instructed to include the following phrase in their prayers: "for a successful surgery with a quick, healthy recovery and no complications".[42] Some participants complained that this mechanical way they were told to pray as part of the experiment was unusual for them. Complications of surgery occurred in 52 percent of those who received prayer (Group 1), 51 percent of those who did not receive it (Group 2), and 59 percent of patients who knew they would receive prayers (Group 3). There were no statistically significant differences in major complications or thirty-day mortality. In The God Delusion, evolutionary biologist Richard Dawkins wrote, "It seems more probable that those patients who knew they were being prayed for suffered additional stress in consequence: performance anxiety', as the experimenters put it. Dr Charles Bethea, one of the researchers, said, "It may have made them uncertain, wondering am I so sick they had to call in their prayer team?'"[43] Study co-author Jeffery Dusek stated that: "Each study builds on others, and STEP advanced the design beyond what had been previously done. The findings, however, could well be due to the study limitations."[44] Team leader Benson stated that STEP was not the last word on the effects of intercessory prayer and that questions raised by the study will require additional answers.[45]

The science experiment would probably not work so well against them, because they are usually already biased against science. You can usually find out real quick just using one word. Evolution.
Interesting study though. I wonder what the outcome would be if you told them that you would hold mass for them during the operation, just in case they died while it was happening . . . . I guess we will never know the answer to that one, ha ha.