Being mindful of those contributing factors I mentioned previously, that's generally true OZ!
However since "mild" infectious processees often clear spontaneously, they can readily be dismissed as "part of the deal" by users.
Consequently what can happen I eluded to earlier as an "additive effect", in that several injections in relatively close anatomic proximity can overwhelm host immunity with the end result being an overt infection.
@synical there is one question you "friend" could ask his Doc that MAY help differentiate whether this infection is the result of contaminated gear.
Have him ask the Doc WHAT bacteria were noted in the CULTURE!
Although some docs choose not to perform a wound culture esp if a "sterile abscess" is suspected and the latter would be a frequent occurance IF antibiotics were prescribed BEFORE the culture was obtained.