No guarantee it won't bootloop !!! I did on my note 3, you just have to take a step back and keep going. Let me give you an overview, but then you need to follow the individual steps in my first post for each part you need to do.
Ok, the first thing you are going to do is BACK UP ALL THE DATA ON THE DEVICE !!! I.E. music, pics, and videos. These are found on the internal sd card copy and paste them on the desktop of your PC.
Now download Odin to your desktop and extract it. (get the link in 1st post) Download a fresh copy of NH7 4.4.4 to your desktop (get the link in 1st post) and extract it. Now follow the instructions in the first post to flash a stock rom. Let it boot up, if it bootloops take off the back and pull the battery. Let it sit for two minutes, replace the battery and hold the power button, volume up button, and the menu button at the same time. It should boot into a stock recovery, if it DOES use the volume button to navigate to factory reset and push the power button. Let device reboot this will take a couple of minutes. Now go to settings/general/about device and see what version of KK you are running.
If it DOES NOT boot into a stock recovery, and goes back to your custom recovery or bootloops. No problem, pull the battery let it set for a couple of minutes reinstall battery then hold the power button, volume down button, and menu button at the same time and boot into download mode. Go to the first post and flash the rom again it will stick the second time.
Once you are back to bone stock 4.4.4 NH7 just follow the first post for rooting and recoveries. I know this is your friend's device and you don't want to mess it up. So before you try anything check here
http://autoroot.chainfire.eu/ go to SM-G900P NK4 and try the CF-auto root for the s5. The s5 and note 3 are pretty much the same. You can try to flash the root through odin the worst that can happen is it will fail and not install. Then you are back to try and going back to 4.4.4 NH7.