The other answers here assume you’re trying to run the virtual machine on the iPad processor itself, and they’re right that the power isn’t there. However, you can run Microsoft RDP (among others, but that app is pretty good) and connect over a network to a Windows “machine” running somewhere else.
If your iPad user doesn’t have a way to connect over the network, the rest of my answer here won’t be of much help.
“Somewhere else” could mean you run the Windows inside Virtual Box just like you’re doing now and the iPad connects over the network to it. Be sure that the virtual Windows machine is getting an IP address from the network and not being NAT-ed behind the network address of the host machine. Inside the VM make sure you have the Windows system configured for remote desktop (here’s a link: https://www.windowscentral.com/how-use-remove-desktop-app-connect-pc-windows-10-0 ).
And if your user has decent Internet bandwidth, the same principles could be used to connect to a virtual machine running just about anywhere. Amazon’s has a service called WorkSpaces cloud deskop solution is free for a few months tto new AWS accounts.
It’s a little odd at first to navigate a desktop interface on a touch screen, but it works reasonably well.