Yosemite would be a significant change for a 79 y/o person.
The application upgrade may change how one uses them to
perform similar tasks, in new OS X compared to older OS X.
The Snow Leopard 10.6.8 is a great system, unless the senior
citizen has some great need to procure the latest iPad, iPod, or
iPhone, and get all those Apps, and some other stuff.
The older computer likely is not up to the upgrade. At least 4GB RAM
for the operating system, a Core2Duo processor, & build year series
of fairly recent manufacture, are required. The ability to upgrade the
hardware (more RAM, larger/faster HDD, etc) is an important factor.
If the person is happy with Snow Leopard and has no need to get
into a later OS X, what is the rush? The older computer could be
upgraded to run Snow Leopard better, without loss of the users
original files, by adding an SSD and more RAM. And backup of all
the user files to an externally enclosed hard disk drive that has it's
own power supply. Learning how to backup & restore is more of an
issue with any Mac. And the process has changed in later Macs
that use the internet for most software update & restore efforts.
So it would be up to the vintage user, not the sales person, to choose.
If the old configuration were left intact, for that person's use, and they
also got a newer computer with later OS X, that would be different.
Then they'd not be forced into some unworkable new situation with
no way back to what they had. I'd not want someone to upgrade me.
In any event...
Good luck & happy computing! 🙂