My past experience as an IV user I never had an abscess like that unless I was actively or recently using drugs (as Vaya pointed out you didn't say what his Drug of Choice was so again this is my experience as an Opiate user). Of course I had tons of excuses to try to explain them away (spider / bug bites probably being my favorite). I am not sure what his excuse for it was. Yes people do get abscess for other reasons, but putting 2 and 2 together (addict / location of it) I would say it doesn't sound good.
As far as location yes it does go in that vein opposite your elbow, but most addicts don't stop there. Arms, hands, legs, even feet. Left hand versus right hand eventually doesn't matter all that much either. Addicts find and use what is available to them.
As Vaya said not a ton you can do and letting it consume you doesn't help. Just makes you crazy. Take Care.