Peoples bodies are different... metabolisms are different. For some people, they need less then even 24 hours... others need days. The length of time you are on it has a big impact because of it's long half-life..it builds up in the system and your body gets very used to it. Has your friend been on bupe prior to the two injections or was it just a one time thing? For me, unless on an outrageously high dose of bupe, 24 hours is all I need.. even if I've been on it for a while, but that's just me. Also, I was using heroin, much stronger than percocets so whatever blockade was remaining was easily pushed through... that might not be the case with a weaker opiate.
Edit: Did your friend IV the bupe or IM it? In your thread title you say IV but then say IM in the post... there is a big difference between 2mg's IV'ed and 2mg's IM'ed..