How long must I wait for a GM to get back to me about a report?

[Absimiliard]

GMs aren't allowed to tell you if/what action is being taken. If you hear anything at all, it'll be "I'm sorry you experienced <insert thing>" along with some other platitudes. Odds are it'll take them a while to get around to it anyway, assuming enough people reported the dude for them to notice in the first place.

Usually they won't ask for extra details if you've already supplied them with information such as time, instance, etc. They're able to look into it themselves for the most part.

As for your friend getting a prompt response; it depends on what the report contains. Certain words will get you their attention a lot more quickly. ...Or your friend was just really lucky. That sometimes happens too.