we will need more information than that to be able to answer you correctly. did you create bootable usb drive from the win8.1 iso ? did you setup the bios correctly by putting your usb drive in first boot ? did you disable "secure boot" option ? if no, do it.
To enable/disble EFI on the 15R: Press F2 during boot, using the arrow keys press the right arrow key until "boot" is selected. Press the down arrow key until "Boot List Option" is selected. Press Enter and select "UEFI" to enable or arrow up to "Load Legacy Option Rom" and press enter, set to disabled. You may need the latest bios installed as it was a known problem on older bios versions.
title of the thread? yes you should see a line on the right of the title area called Thread Tools, click that and you can edit the thread title.
are you sure that he needs to flash the bios with a more recent version to be able to install win8.1 on his machine ?
the latest bios fixes the problem of not seeing the option to change the UEFI/Legacy option, it not always show on the older bios's. That way he can make sure it is in legacy mode, although W8.x will install in UEFI mode and secure boot on/off.
i was sure you were going to say this i don't even know if your bios needs to be updated from your windows session or from the bios with the needed utility.
i believe the DELL bios's are all EXE format now that contains all that is needed to update the bios. It will say if the bios is older and can be updated or if it the same version you can cancel.
OK, so try the F2 way as mentioned in my post above and make sure it set to Legacy, but win8.x and 10 can install in UEFI mode anyway so that not be a problem, the USB needs to be FAT32 for UEFI to 'see' it. Legacy can be FAT32 or NTFS. If you used rufus to create the usb from the iso then you might need to redo it with the BIOS or UEFI mode selected.