I would think that a 7" tablet would be the way to go and there's slim pickings in that range right now. I also wonder if there's a tablet out there that has a screen that would function very well in direct sunlight applications. It also strikes me as pretty extreme to think about using a tablet as a GPS device unless it's more of a mapping device for route planning rather than route following. One more random thought - in order for any GPS to be useful it has to either carry all of its maps on board or have a data connection. Google maps only offers the option of download on demand (no real local map storage) and therefore requires a data connection to generate a route. You can, if you think of it ahead of time, plan a route and allow it to download the necessary maps before breaking the data connection (in the case of a wifi-only tablet) but otherwise your friend would need to be looking at a tablet with a built in 3G or otherwise modem which is going to make it a pretty expensive GPS.
And Ken, the software requirement is simply listed there to show that if you want to attach the tablet to a computer (as a USB drive, for example) that it is compatible with all of the major OS's.