It was definitely not a clean Windows install. She inherited this laptop from someone else, and it wasn't well "cleaned up" (some previous user settings were still there). Also, the IT had put some restrictions on how Hyper-V could be used, I think, or maybe have pre-configured it somehow (most likely unintentionally, when configuring something else).
The sort of problems it was having were related to virtualizing the network adapter. Somehow after about half an hour it would just "stop working". Since it's Windows... no debugging, no logs, and the Web search brings up a lot of nonsense when you try to figure out the problem. The kind of "stop working" was that the host could still use the adapter, but the guest system (the one running Docker) would still have an IP, knew the IP of the router... but sending anything to the outside world (i.e. the router) would end up lost.
Maybe it was a faulty adapter. Maybe faulty Windows driver, or maybe a problem in Hyper-V / its settings... I never figured it out. The same problem existed if I tried to do it with any VM I'd create there, if I tried to virtualize the adapter, so it wasn't unique to Docker. Eventually, I've given up on the idea of having a virtualized adapter, and put the VM in its own NAT'ed network.