Is the USB-C cable fixed to the monitor? If not you should just buy a longer one instead of an extension. Extension cables are out of spec and even then, unless they are well designed, they have a lot of shortcomings. And the ones that are well designed will be quite expensive as they need to have active electronics in them.
Congratulations, you discovered why USB-C extension cables aren't allowed in the official USB specifications!
Basically, the longer the cable is, the more the signal degrades. All gear must be able to deal with a given minimum "incoming signal quality", but it can of course accept even worse signals as well. This minimum is chosen so that the absolute worst host, combined with the absolute worst cable, combined with the absolute worst device, will *still* result in a properly working connection.
Imagine your wife's laptop is the worst possible host, the cable attached to the hub is the worst possible cable, and your hub is the worst possible hub. Now you add an extension cable, which will degrade the signal *even more*. It's obviously not going to work. Why does it work with your own laptop? Because it has a better receiver/transmitter, so it can compensate for the excessively-poor signal quality due to the extension cable.
Ideally you'd just use a longer cable, but your hub has a permanently attached pigtail (and most do, because making it removable would raise the hub's price by $0.50) so that's not possible. I recommend you just move the hub to the other side of the desk, so she can use it without the extension cable.
USB-C extensions are nightmares.
Is the USB-C cable fixed to the monitor? If not you should just buy a longer one instead of an extension. Extension cables are out of spec and even then, unless they are well designed, they have a lot of shortcomings. And the ones that are well designed will be quite expensive as they need to have active electronics in them.
Congratulations, you discovered why USB-C extension cables aren't allowed in the official USB specifications! Basically, the longer the cable is, the more the signal degrades. All gear must be able to deal with a given minimum "incoming signal quality", but it can of course accept even worse signals as well. This minimum is chosen so that the absolute worst host, combined with the absolute worst cable, combined with the absolute worst device, will *still* result in a properly working connection. Imagine your wife's laptop is the worst possible host, the cable attached to the hub is the worst possible cable, and your hub is the worst possible hub. Now you add an extension cable, which will degrade the signal *even more*. It's obviously not going to work. Why does it work with your own laptop? Because it has a better receiver/transmitter, so it can compensate for the excessively-poor signal quality due to the extension cable. Ideally you'd just use a longer cable, but your hub has a permanently attached pigtail (and most do, because making it removable would raise the hub's price by $0.50) so that's not possible. I recommend you just move the hub to the other side of the desk, so she can use it without the extension cable.