Not to downplay, but this is how my last glute injection went as well (the last time I shot glutes).
I was shooting a higher concentration product and probably shook the needle around a bit trying to twist back and shoot my glute. I really think the combination of higher concentration products and not holding the needle still causes this infection (I've had it on my delt once too).
Got all red, warm, etc. I immediately started a course of Zithromax 3-day (5-day will work as well), and it cleared right up.
On something like this, timing is extremely important. The Z-max hits hard and fast. Do you know what type of antibiotic you're buddy is getting you?
Realistically, you need to go to a doc if this doesn't clear up straight away or if it worsens to any degree. If you get Z-max on board right away though, it might make it go away. You'd know within a half day or so ...
I don't do any sites now that I can't control perfectly (aspiration and injecting) and I use 25s to shoot with now too. Quads and chest are the only places I shoot now.