You could go the NAS route, but I believe that would be a bit overkill in this situation, unless you want easy access to the content of the NAS drive from other computers than your wife's Mac as well.

If not, I would simply buy an external drive, this would be the quickest, least-hassle and cheapest option in any case. 2.5" models also don't need an external power adapter ; they draw their power from the USB socket, so no cable clutter as well.

They come in capacities from 500 GB up to 2 TB, so I imagine that would be enough. I particularly like the MyPassport models from Western Digital.