If she’s just using it for recovery, I’d say go for a cheaper one. Anything with power (bar the top end ones) is a bit of a waste of time because they don’t use a proper strain gauge. If she’s planning on intervals, or zone work then maybe a decent bike computer to measure cadence and HR is worthwhile, but if she’s spending a lot of time on it the biggest factor will be boredom.
The other alternative is rollers – she’ll have to use her core a bit more which could be a good or bad thing, but they’re more ‘interesting’ to use and it actually feels like you’re riding as oppose to just sitting and spinning.
What bike is she doing it on? If its MTB, then you’ll need either a slick/turbo tyre or a turbo that works off the rim.
My suggestion would be
Cheap turbo off of Ebay <£80
Slick tyre C£20
Lots of DVD’s/Downloads off iTunes/TV