He should at first assess his needs for backing up files. What kind of data for home use could possibly fill 20TB? Does he need to keep a backup of everything?
Either way, you could always get a second (third?) NAS server and HDDs and setup automatic backups.
A NAS with 2 bays will set you back 100$ or less. Add 2x 4TB for 400$.
Better yet, build a custom file server for under 200$ with a nice case with plenty of room for disks. Be sure to choose a motherboard with 4 or more SATA ports and fill it with HDDs. FreeNAS is a great OS with many options. Five 4TB (=20TB) would cost under 1000$, so for 1200$ you have a nice long term solution with great flexibility.