This is not that uncommon with underground gear, they are poorly manufactured steroids which really is something only qualified people with equipped labs should attempt to do, not meat heads or chinkies in kitchens.
UG gear is more prone to crystalising inside the muscle. In addition to this, sometimes the dosing of the alcohol is too high which can cause muscle inflammation, or the type of oil used not being suitable for intra-muscular injections. Remember all UG labs buy their compounds to make the gear from China. China is notorious for poor reliability and counterfeit and corner cutting procedures....fuck even Aston Martin had to re-call thousands of cars as their Chinese DuPon plastic supplier turns out was using fake DuPon plastic....yet here we are injecting ourselves.
Take Ibuprofen as it is an anti-inflammatory medicine, if it doesn't get better go see a doc ASAP, a course of anti-biotic might prevent further aggaravation of the infection, he might end up with puss in his glute like this Bodybuilder below which had to be operated on to be drained, leaving a massive gaping hole in his ass (no pun):
