You could also buy a spin bike and if she wants to do peloton still via an ipad she can do that for $20 a month. This way it is a win for you and her. I believe this way it just does not relay the power / cadence stats to the screen.
I don’t have one but my friends do and they love it. I tried it a couple of times but not for me and prefer TR
Similar situation here - my wife is using an old Mountain Bike on my Elite Direto that is fitted with a 11spd ultegra cassette. The mountain bike has an old 8 speed deore groupset. It does not work flawlessly, but there are 2 or 3 gears that do work, without chain rub.
So she’s not allowed to touch the shifters, but her workouts are in ERG mode anyway 
To be on the safe side (and not mess up the drivetrain of my road bike) I use a different part of the cassette, but this could be unnecessary.
I use the Stages at LifeTime Fitness. Pairs easily to iphone, consistent watt readings from bike to bike. Do 3-4 TR workouts a week on it
You don’t say much about how your SO wants to use it, but a spin bike is better than a bike on a trainer. I’d dig into why that is. Who wants it to have a power meter? Does a bike that reports wildly unreliable power data (like pretty much all of them that estimate power from flywheel speed) okay? Is something like the Stages that measures force directly but only from one side okay?
I think the answer could vary a lot depending on what the goal is.
Thanks for all the responses everyone!
I like the Stages ones as well, but they’re at the premium end for the most part. After some looking, I settled on this one I got on AMZ, “EFITMENT Indoor Cycle Bike, Magnetic Cycling Trainer Exercise Bike w/ 44 lb Flywheel, Belt Drive and LCD Monitor with Pulse - IC033”
Heavy fly wheel 
Belt drive
Head unit
Integrated iPad holder
Magnetic resistance
The last thing was the hardest to find on a budget bike. This one was ~$450. It’s perfectly quiet and between the belt drive and the mag resistance it will almost never need maintenance.
Two complaints that would be easily solved in a more expensive model: it’s “dumb”, so doesn’t connect to TR, Z, or Strava directly, and the head unit isn’t backlit. To the first point, I have some old wahoo sensors I might strap on. To the second point, I’m going to see if I can get a different head unit, which fingers crossed might be able to connect to a phone or iPad on its own. 
Thanks again everyone.
Sorry, way late to this thread. @dotsonrk - sounds like you found something that works and fits the budget.
I was going to recommend a Schwinn. They are really solid spin bikes and easy to look after. You could throw a set of power meter pedals on it and it would still be a pretty good bike to train on.
I have a Schwinn that my wife still uses and I used to use before getting into other trainers. There used to be a company called Studio Cycles and there was a guy called Jeff Wimmer who would fully assemble them, test and optimize everything, and then send them out. You could also call him up anytime and he help you out. Amazing dude. Unfortunately he passed away in 2013.
http://www.indoorcyclingassociation.com/blog/our-industry-loses-a-shining-star-rip-jeff-wimmer/
For cadence, the wahoo RPM can just be attached to a shoe, so it’s super easy.
For power, as others have said, whatever built in PM it has is probably niether accurate or consistent from session to session. Any speed is also going to be meaningless. It sounds like she may not care about power? By nessesity I do a bunch of workouts with just HR and RPE on spin bikes, and it definitely works fine. You could also definitely go with pedal based PM, or maybe even crank based (i’m not sure if the cranks are the same)
Also, the stages sc3 is great if you have to use a spin bike. Super sturdy, decently adjustable, and have a legit power meter - basically a beefier one sided StagesPower
There are many best indoor spin bike actually. Such as the Sunny SF-B1001 home bike, the Keiser M3i one, or the Harison B1850.
Not sure where you are but I have a Keiser M3 that I’d be happy to sell at a very reasonable price
Our local gym has Keiser M3i (as mentioned above) which I found pretty good before I had a smart trainer at home. If you care about recording workouts (inc power output) then get the M3i, not a regular M3 - then you can record via Android app (there’s probably an iPhone app as well). Note though that you can’t control the M3i via apps like TR (if that matters).
I have the Keiser M3i and with a converter you can link to TrainerRoad/Zwift and other platforms. I do most of my indoor training on it and it is working fine. No Erg Mode though. When indoors I switch between this and rollers. I bought new and like it because the whole family can use it. I also use Power Pedals that I use outside. If using power pedals you do not need the converter.
I did some research for my niece who was on a similar budget and we landed on the Schwinn IC4. The Keiser was well out of the price range.