Sometimes you can get proper old retail install disks from Apple User Groups. Many such groups have a member or two who tend to archive old software. Start here:
https://appleusergroupresources.com/find-a-group/
Lion only runs on Intel-powered Macs. eMacs are PowerPC.
Your processor speed determines what OS you can install. If the eMac has a 700 or 800 MHz processor, its max OS is 10.4 "Tiger." If it has a 1.0Ghz or faster processor, its max OS is 10.5 "Leopard."
Many retail install disk are on DVDs but many early eMacs do not have DVD drives, only CD drives. Make sure to order the proper media.
Does the computer function OK now? I ask because most 1.25ghz eMacs have defective logic boards and they won't start from the HDD or an optical disk. If that is the case, buying expensive disk sets will be a waste of money. The known-bad units fall in these serial number ranges:

The serial number is inside the opticals drive door:

If the serial number starts with "R" or "RM," the unit is an Apple refurb and those number ranges won't help.
This chart shows how to determine which of the four model families of eMacs your Grandfather has:

Config codes are the last three characters of the serial number.