You can confirm this for yourself, but I suspect that the lug width of the watch is 16 mm.
One option, as Pete says, is to get adapters (search for “G-shock adapters”) that will connect the watch to a more standard width (usually 20 mm or 22 mm) NATO strap.
A second option may be to get a 16 mm NATO style strap. This should attach directly to the watch without the need for adapters, but may be a bit narrower than some folks prefer.
A third option might be a third-party strap specifically designed for G-Shock watches, but not an OEM part. These seem to be pretty reasonably priced.
At the risk of stating the obvious, you’ll want to verify that the strap length is sufficient for your wife’s wrist as well, keeping in mind that the last hole on most straps is some distance from the end of the strap. Probably not an issue for most people, but I have larger wrists and also don’t like a super tight fit, so I have to be careful when ordering watch straps that I don’t end up with one that is too short.
Please let us know what you decide and how it goes!