Migration Tool For Mac Book Pro
If you just have Documents or Downloads, then it might be worth using a USB. If you have Outlook, or one of the other listed mail clients, browser settings and a desktop background, you'll save time with Migration assistant.
It will be faster over ethernet than over WiFi. I would even connect the 2 with a crossover cable and set static IPs just to make it faster, especially if your wife has a lot of data.

Apple Kit Customer Data Migration Tool For MacBook Pro 2016-2018 June 29, 2018 by Apple Gazette 1 Comment This kit has been around for roughly two years – ever since the version of MacBook Pros came out containing soldered-in SSDs. The lowest-priced brand-new, unused, unopened, undamaged item in its original packaging (where packaging is applicable). Packaging should be the same as what is found in a retail store, unless the item is handmade or was packaged by the manufacturer in non-retail packaging, such as an unprinted box or plastic bag. Customer Data Migration Tool connector on 2016 MacBook Pro logic board While not fail-proof, the tool is a convenient, last-ditch option for data recovery when a MacBook Pro's logic board goes kaput. Apple Kit Customer Data Migration Tool For MacBook Pro 2016-2018 June 29, 2018 by Apple Gazette 1 Comment This kit has been around for roughly two years – ever since the version of MacBook Pros came out containing soldered-in SSDs.
Make sure the Mac can ping the PCs IP. Make sure you're sharing your files from Windows. You can double check by connecting to the Windows system over SMB by opening Finder, then press command k. Enter The Windows PC's IP address and you should see file shares. If you can't see and connect to the shares, then it won't work.
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@gromack - yes it sounds like you're missing something. Are you running Migration Assistant on Windows as well as OSX. You need to run the Windows version first so that it broadcasts its availability in a method that the MacBook can pick up.
Download, install and run Migration Assistant on Windows first - This describes the entire process. If you've installed Migration Assistant on Windows and its running and the OSX version cannot detect you're then looking at a network WiFi issue or Firewall.
As your wife has a new MacBook you should be able to use a standard Ethernet cable to connect the 2 laptops directly (a crossover should not be necessary) so that is one possible option. If Ethernet cable is not an option and you're on WiFi. Ensure that you Disable any AntiVirus or similar programs Disable the Windows Firewall if it is active Please report back if you've done ALL of the above and it still doesn't work. It is worth persisting with Migration Assistant as it does MUCH more than just copying files from one PC to another and will save a fair bit of time compared to using an external USB drive and double copying.
Macbook Pro Migration Assistant
Ok, I was too quick to close this, thinking I had it all under control. As it turned out, just copying files to a USB drive was the best solution. The migration process started & then would crap out at some point.