Has the gun ever jammed, failed to eject, failed to feed, failed to fire, when you were shooting it? I know this term is thrown around a lot, but maybe your friend was limpwristing it. It's more likely to happen with the dual guide rod assembly and weaker ammo.
If it has done any of those while you were shooting it, it could be a problem with the extractor. Failures to feed and failures to eject together usually indicate that the extractor doesn't move freely enough. I'd try replacing the extractor.
Also, what ejector does the gun have? Is the ejector stamped 336 or 30274? 30274 is the latest for Gen4, it can help prevent failures to eject and erratic ejection. If yours doesn't have it you can have a local Glock Armorer order it and install it for you, or you can call Glock. Glock will tell you to find a local Armorer, if you can't do that you should tell them that and ask them for a prepaid shipping label so you can send it back to them and have them install it. While they have it they'd check the fit of the extractor as well and replace it if necessary.