Unless there's a special reason that it HAS to be a Due, or (shudder), Pi....
Why not give relative a "mature", stable, does-most-things-Due-does Mega? (Others may know better, but, from Canada, I would go first to Sparkfun.com (Colorado) for any of this sort of stuff. Not always the VERY cheapest... but big enough to be reliable, to have a reputation to care for, good stock levels, etc. Prices usually good, too.
(I'd be VERY happy with a Mega, if you can't get one of the others in time! <^_^>)
Seriously....
For the Very Keen, the "latest and greatest" is, of course, "necessary"... but if you just want to support (or maybe inspire) someone's interest in this line of "play", what is wrong with Mega? It was the "must have latest-and-greatest" not so long ago. (I am still on PRE-Mega Arduinos, and have LOTS of fun! And I'm no newbie... see Arduino Page- Main Sheepdog Guides Page)
I admire the Pi team for "having a go"... and a part of me wishes them some luck. But why dump your relative in the land of the "getting started" when the Arduino is so well advanced? The Pi is NOT "this generation's BBC Micro". It is NOT "very simple". Dig around the net for "getting started with your Pi" articles, and for the "these are the things you need to get right" and the "this is how you can fry it" articles....