Your buddy is a real fuckin cocksucker... With friends like that, who the fuck needs enemies. Seriously dude if i were you I'd beat the living shit out of him with a crowbar for doing something so bloody stupid.
I think 1 of two things could have happened. He may have hit a nerve, which in some cases is not something to worry about, however it most often is. But if you're not in the most EXCRUTIATING pain at this point, or if you notice that there is a slight improvement in mobility and pain, I would imagine that this is simply a case of a fresh muscle feeling the effects of an oil based intra-muscular injection. And since sustenon is known to hurt like a bitch, and that leg injections arent commonly the first place any newbie would ever inject, this could just be your bodies way of reacting to it- im sure many if not all of us have been there. After a while you build up a thick skin injections, but that's definetly a no no as far as first injections go IMO.
But on a side note, dude you havent felt pain unless you tried injecting the Bulgarian Winstrol that was circulating around 5-6 years ago- The shit gave me nightmares, fevers, and pain beyond all pain…
But damn I looked great.