Sadly, no lock is going to stop a determined thief. If you know of one, please let the rest of us know. Some will destroy a frame in order to get the components, so there really is little you can do.

If theft is really that much of a risk, I'd lean toward not giving her such a nice bike. I still see some decent bikes at decent prices, even though bikes have gotten scarce during Covid. I'd go that route personally and if it gets robbed, you're not out so much.

I do know someone that covered a bike in black gaffer's tape. He did it mainly for protection from rocks and such, but it is always an option. But like others have said, some educated thieves won't be fooled. So it is a crapshoot.