Or I can tell you how to install it using a PPA on Debian, involves installing software-properties-common first.
With Stretch, you can either download the. The Tessa MATE and LMDE 4 'Debbie' already have Timeshift installed, of course. I would suggest 50 - 100 GB at the end of the drive, where it can be expanded on using GParted if need be. If I might suggest, if your friend's HDD has spare capacity, it could be good to set up a discrete Timeshift partition that could be used for all 3 Distros.
