This is actually a common idea in my area (Alabama), and it's somewhat difficult because it's proving the absence of something, rather than the presence of something. That said, there are no historical records of the use of "jamblers" in golf courses form the United States, or other parts of the world for that matter, and it seems unlikely that these "Jam-Boys" were ever used. They are probably simply an urban legend and nothing more. It seems like there are much better, cheaper ways of attracting flies away than ordering some poor kid to stand in a field covered in jelly. For instance, one could cover a tarp in jam and place it on the ground or around a tree. A person seems wholly unnecessary for this. Even then, when has a golfer's game been ruined by the ever present swarms of flies that inhabit golf courses? Flies aren't that big of a issue. It just seems like an overly elaborate solution for something that shouldn't be a problem. The lack of historical evidence and the presence of common sense makes the likelihood of jamblers existing quite grim. It's also worth noting that the Wikipedia article for Jam Boys was deleted as a hoax in 2009.

Furthermore, after a cursory internet search, I found that the details of this idea differ from forum to forum, with some people claiming that jam boys were used in South Africa, some claiming that it was England, and others claiming that it was the southern United States. This is very much like many common urban myths that vary the location of the event (e.g., "My cousin out in California/Oakland/Milwaukee/Huntsville/Charlotte said that a girl at his school was attacked by the Goat-man with a hook for an arm that was flashing his high beams in her rear-view mirror). This seems very much like an urban legend with no historical precedent. I think it was just a local trying to pull your friend's leg.