Just been down a similar route for my (non-cycling) wife and in the end decided on a FitBit. What it doesn’t do is distance – but can estimate from the number of steps and estimated stride length. Will record heart rate, steps and on the app provides an estimate of calories etc..
But, I can’t see how, other than measuring effort, it will add much to being on a bike. But if she has an iPhone she could use a Strava or other app for distance/speed/elevation?
But playing Devil’s Advocate on the original question, how do you know that your calorie count isn’t the one that’s inaccurate? I see these devices as fine for comparing your own day to day effort, but not for comparing with someone else unless you both have the same devices