How to get a 7-speed bike with a freewheel on a direct drive trainer

As Daniel Hicks said in his comment, it doesn't matter what's on her bike's wheel. The problem will be that her drivetrain is set up for 7 speeds, and you'll be putting an 11-speed cassette on the trainer. So shifting for her will be rough at best.

With a smart trainer, if you only ride it in erg mode, it doesn't matter what gear you're in. So if there's one gear on her bike that runs smooth when on the trainer (there might be, which would be a lucky coincidence, otherwise you'll need to tweak the barrel adjuster every time she changes between the trainer and the rear wheel), and she only uses the trainer in erg mode and always shifts into that one gear, then you don't need to do anything at all.

The other options are:

• upgrade the drivetrain on her bike, and it sounds like that will include upgrading the rear wheel. (It might actually be cheaper to replace the whole bike.)
• change the trainer between an 11-speed and 7-speed cassette every time you swap bikes. I believe that 7-speed cassettes and freewheels all have the same spacing.

One option costs more in money, the other in time and annoyance.

A 7 speed cassette and a 7 speed freewheel look the same to the chain, so you can fit up a 7 speed cassette to the trainer. and then juggle shims/spacers to get the chainline perfect.

I have an old 80s road bike with two sets of wheels, one is freewheel based and the other is a cassette. Works fine, and friction shifters sort the slight misalignment.

7 speed cassettes are less common, but definitely available. I got one from a popular chinese website, but an LBS should be able to order one for you. Get one with the same 7 cogs, as well as the same large and small sized cog. If the intermediate cogs differ in tooth count, that's okay.

Your other option is let her ride your bike - there may be fit issues.