It is probably more likely that this lump could be a result of one of the following two things:
Firstly, it could just be a hardened area of scar tissue. When one injects repeatedly into the same spot, the surrounding tissue hardens and goes kinda gristly. I know that if my good friend tried to inject into the crook of her elbow now she would have a pretty hard time getting the needle through the hard, gristly, sclerosed tissue. Basically, all that trauma of years of sticking a needle into a vein causes the flesh to scarify and harden.
Or this lump could simply be there because as a result of missing a small amount of a hit. As long as an infection has not developed, your friend could have what is known as a sterile abscess. This is basically a small amount of some contaminant or foreign body that is trapped in your flesh. It can take few months for a sterile abscess to be dealt with by your body's immune system, my associate has had a small lump on her hand the size of a pea for a few months now - it's steadily getting smaller, it was the size of a small marble to begin with, but still kinda noticeable.
As long as there is no sign of infection this should not cause too much concern. Localised signs of infection include redness, increased swelling, the area feeling hot, tenderness or soreness. More generalised signs of infection include a raised temperature, shivering, sweating, aching, headache etc. If any of these occur, it would be a very good idea to seek medical advice.
Odd little lumps and bumps unfortunately come with the territory of injecting heroin. It's very difficult to avoid encountering something like this if one injects for any length of time. The best thing one can do is ensure that they maintain a good injecting practice (using new works every time, keeping things clean and sterile etc), and that they pay attention to anything that gives them cause for concern, seeking medical advice when necessary. Just keep an eye on the arm, it doesn't sound too serious to me, but if you get worried, advise your friend to see their GP.
Hope that has helped to put your mind at rest a little,
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