Hmm for an absolute newcomer to FF (and JRPGs in general I'm guessing?) I'd recommend FFX with the Standard Sphere Grid. I'd say FFVIII is easier if you quickly grasp how to use the Draw/Junction system to your advantage, but if you don't, the game can be pretty frustrating at times. FFX doesn't have this problem so long as you at least get how to advance and activate nodes on the Sphere Grid, and the game paces its difficulty spikes well enough. And finally, FFX has no element of time pressure in its battles.

I do not recommend them going into the postgame of the game unless you they enjoy it though.