El Capitan stuck on install
Apple Support: 1-800-275-2273 (Or you can get it from here)

Call, it wont take long, and you don't need to have applecare or anything like that. And this way, if something breaks, maybe you'll be eligible for some kind of reimbursement.

You can do the whole thing with your phone on speaker. I actually tried to call them myself (and then relay the information) but one of the first things they ask for is a serial number. 

The occasions i've called them are all blurred together, but i vaguely remember being told to hold the power button for some specific length of time, holding some key while it started up (which might have been to activate disk utility), and unplugging the iMac. 

The key thing, though, is that i called them, it didn't take long, and in doing so i was able to fix it.