If 10 minuets have passed he is mostly ok, for now. Nothing immediately threatening. But you need to be careful as something still may be very very wrong.
However, IV drugs brings a variety of risk. Look at the spot he shot at, does it seem damaged? Any lumps? Is it inflamed? He may have just tweaked and hurt himself physically with the needle, and maybe missed just a tiny little bit. It happens, especially when on coke. I've fucked a vein up with one of the larger shots I did, using a 0.5ml rig doing probably the strongest shot I have, at around 20 unites left I tweaked and missed half what I had left and the way I did it IT HURT LIKE HELL, thats how I knew to stop and did the rest into another vein.
I had a huge panic attack once, I shot a D and my whole arm went numb and had a feeling like a morphine rush but painful. Like my arm had fallen asleep, it was bad. I felt it during the D rush, and thought it was strange as it didn't hurt until the rush war off. It felt like a painful morphine rush, I don't know how else to explain it. Went on twice as long as the rush from the D. I was freaking out, I was told by junky friends it was a circulation issue but I thought my arm may be falling off or some shit, LOL. It was kind of numb for like 10 minuets after the pain wore away, and very red for a few hours.