If she is used to riding tri, look for a tandem with a looong stoker compartment. Most old tandems like my Super Galaxy are relatively short. Cannondale would be a good choice used.
NICE tandems are not cheap. So my advice is by decent used and see if the bug bites. They provide a different flavour of cycling best suited to two riders of different abilities. My Kiddyback which has a 12″ rear being the obvious example.
Finally, it is a closely guarded secret that the stoker gets the bettet ride – the captain balances and steers and seldom enjoys the view. And the stoker is always right!
If you want something different look for a Haze Pino or Bilenky Counterpoint in the US.
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Daren – also a tandem geek! There I’ve come out to the forum 😉