I have a light post at the driveway. They are about 2 feet by 2 feet. The rock is separating how can i repair this quickly without taking it down? Help please
Well, the site may desire long complicated answers, but in this case "You can't" is the correct one. The only (if any) hope for that thing would be to tear it down, chip off the remains of the failed mortar, and build it again.
You could try all sorts of cockamamie things with polyurethane glue, etc. but they will ultimately fail, while wasting time and money.
Hmmm, you have just found out why all stone masons are 60-year-old Italian guys who have had the knowledge handed down to them father-to-son going back to Roman times.
There is only one option here: call a 60-year-old Italian mason and tell him to bring a sledge hammer and chisel when he comes. When he sees your light, he will chuckle.
As a temporary fix, you could try grouting it with a grout bag: http://homeguides.sfgate.com/use-grout-bag-mortar-joints-25409.html
But I agree with Ecnerwal - it's at best cosmetic (and will probably look still like crap) and temporary. Whatever caused this to fail to begin with will cause the new grout to fail as well.
What did cause it to fail anyway? My guess would be a truck backed into it.