Praying is for the one who prays
Of course, this is all dubious because God would know that He was being used for a test of His powers and wouldn't play that game, but scientists nonetheless attempted to find out if praying for someone else's health has a positive impact.

The researchers enlisted 12 congregations of various Christian denominations, Jews, Muslims and Buddhists around the world to pray for some of the patients, giving them names, ages and descriptions of the illness. The researchers then divided the patients into four groups. The first quarter had people praying for them. The second quarter received a nontraditional treatment known as music, imagery and touch (MIT) therapy, which involved breathing techniques, soothing music, touch and other ways to relieve stress, such as calming mental images. The third group received both prayer and MIT, while the fourth received nothing.

The researchers then followed all the patients for six months to see which patients suffered serious complications, were re-hospitalized or died from heart problems. Overall, there was no difference among the four groups, the researchers report in Saturday's issue of the Lancet medical journal.

So, it can't hurt. And there's reason to believe that, like meditation or relaxation, it can be healthy for the person doing the praying. Not sure if that's ironic.