depends on how much she's going to use it. I bought a 2nd hand Ron Thompson for £40 earlier this year and have used it 4 or 5 times so far and can't complain about it.
I don't know how it compares to others on the market so it might be worth doing a bit of research on them but it the does the job for me at Tal Y Llyn. GAC doing a deal at the moment:
Glasgow Angling Centre – Float Tubes
some good advice can be had at the British float tube forum:
British Float Tube Association - Index
hope this helps