Haha you guys are funny. I'll tell you what your friend did. He looked up some weather information saw that it was going to rain and then to amaze you he told you about his rain spell and claimed he did it. It's real easy to trick people with that one.
lol, yesterday i was siting in the stand with my friends, and it started to pour, i laughed and said 'rain stop, this is my rain dance' and i waved my hand back and forth a few times. i was goofing around, and my friends got a laugh. but it immediately stopped. it was just a passing clouds, so i have amazing timing, but if i actually did that, that's neat and freaky at the same time because i really wasn't casting, just joking.
if weather spells work or not, i don't know, i've never seriously cast one. personally, i wouldn't cast weather spells, you're playing God at that point, and you don't want to cause a flood or drought or something?
by 'Naturally Magical' you mean is your friend born with abilities? maybe, some people are. your friend should be careful then, you don't want things to backfire in his face. if you mean 'natural magic' i don't know if screwing with the weather would be natural, look into herbs and stuff, work with nature instead of control it.
Playing god? I wouldn't go that far. People use to do weather magic all the time, trying to prevent failing crops or late springs. Depending on the cultures, you have different ways of doing it. Rain dances, sacrifices, offerings, chants...Our ancestors weren't trying to seize the power of god, simply ease the passing of life. And, if weather magic is playing god, then technically all types of magic are an attempt to be god.
While it's likely your friends just had good timing, weather magic is a legitimate study, and if he's interested in it he should pursue it ^^ It is a hit and miss practice, however.