Dual boot Linux Lite 3.4 and openSUSE-Leap-42.2

[gold_finger]

Now the question is how to make sure that both systems do boot after OPenSuse is installed.

Don't think you have anything to worry about as long as you install OpenSuse in same boot mode as LL (which would be Legacy mode).  Only bring that up just in case computer is relatively new with UEFI firmware.  Live OpenSuse may have ability to boot in either mode.  Whatever mode it boots in is the mode it will try to install in.  Make sure it boots in Legacy mode if she's got a UEFI based computer (likely if it's newer than 2011-12).


Typically you'll need to press a certain key immediately after powering on computer to invoke a Boot Menu that lists devices to boot from where you then choose the USB stick.  If it's a UEFI based computer, you should see two choices for the USB -- one referring to UEFI, the other without any reference to UEFI.  Pick the non-UEFI option.


Haven't installed OpenSuse myself in a few years so don't remember steps in its installer, but it likely gives you option for where to install grub boot loader.  If you want OpenSuse to control booting, have it install to MBR of drive (eg. /dev/sda).  If you want LL to remain in control, have OpenSuse's grub installed to it's root partition (eg. /dev/sda2), or not at all if that's an option.  If you choose the latter you'll need to update grub in LL before the choice to boot OpenSuse appears on the grub menu.  Just run sudo update-grub in an LL terminal to do that and next reboot will have it listed.


Food for thought:
You may want to consider making two 20-25GB Root partitions (one for each distro), one Swap partition, and one large data partition to be used by both.  That way any documents, music, videos, etc. that she has can be accessed more easily from whatever one she's booted into.  I posted general procedure for doing just that for someone else here earlier today.  Have a look here.