That was your first mistake, letting her try one out. It happens to all of us, we even bought ours without trying it out so don't feel bad.
Buying a TI is like leasing a car, you are only paying for the short time that you own it, not the whole thing. The TI is probably one of the most popular boats on the market today, and with that it comes from Hobiecat company which is way out in front of every other manufacturer in support and service, and everyone knows and expects this from Hobie. That's why we are willing to pay a little more (you definitely get what you pay for) for a Hobie product over some other manufacturer like Ocean or Wilderness as examples (not trying to pick on them).
Because we live in Florida near the water, there is a kayak in nearly every yard. We have now owned 7 Hobie kayaks and I can give you plenty of examples of their retained value just from my own experience of selling my used boats (we are on our 3rd TI now).
If you look around ( example craigslist) in an area like Sarasota near the water in sunny Florida where there are lots of boats you can find old used Ocean, and Wilderness kayaks for as low as $50 bucks, basically they are a dime a dozen, and many people buy cheap kayaks thinking they are going to go out and use them all the time, well it's more work than they expected and they end up just sitting in their yard or their garage for a couple years baking in the sun and they are trying to get rid of them, that's pretty much human nature. Now with Hobie kayaks it's a little different story, most if not all Hobie owners are existing kayak owners, and have met or talked to someone with a Hobie and find out all about them (a different world altogether). Most Hobie kayaks are used and enjoyed by their owners on a regular basis (they are already hook on the sport and the lifestyle before ever buying a Hobie).
As a result you will see very few Hobie kayaks on the resale market, and the used ones that are for sale hold their value better than pretty much anything else out there on the market.
Hobie's have a very good 2 yr warranty, where if pretty much anything happens to the boat Hobie fixes or replaces it, this is huge.
As far as the Tandem Island goes the design has changed very little from the original design first released in spring 2010. Pretty much all the 2010 models had their rudders upgraded to the new and improved rudder design (all for free to any registered buyers). Yea there have been a couple minor improvements since then (things like improved AKA bars, improved front AKA cross bar design, and a few other minor things, but it's not worth upgrading most of this stuff if the boat you get doesn't have all the improved features, in as the old stuff worked just fine.
Also keep in mind that the TI has increased in price quite a bit since it's introduction. When I bought mine in 2010 I believe the list price was $4250 brand new, so you had better not pay anything close to that for a used 2010 model. 2010's are now out of warranty so I would be leery about buying a 2010 model unless it was below $2500 bucks, even at that I would be looking it over very closely for problems (mostly a bad or leaky hull, which basically scraps the boat out).
The newer boats 2012 and up are typically around 80% to 90% of new price (depending on condition). This is what I was talking about above when I said it's just like leasing, basically you can buy a new TI for $5500, use it for a year, then turn around and sell it for $4700 to $5000. Your cost of ownership was $500 to $800 dollars (not including accessories of course). I think you can count on Hobies holding their value way better than most boats out there.
I wouldn't be scared to buy a 2010 as long as the price is right. If buying used, my preference would be a 2012 or newer (there have been very few changes from 2012 to current), if you can get one that still has some warranty left, I think it would make me more comfortable.
The most important things to look for are any hull damage and leaks (make sure you physically test and trial the boat).
Any apparent neglect or severe abuse signs (like anything that was used as a rental for example). Small scratches in the hull are normal, and can be fixed and cleaned up, but any very deep gouge needs to be evaluated, and or repaired before you buy it. All Hobies dealers are pretty hand at welding the hulls to repair minor scratches, and Hobie supplies the matching plastic welding rod, and has a plastic welder in their catalog if you want to repair scratches yourself (it's pretty easy).
Buying a used TI is a great way to get a trailer, and accessories like trampolines, scupper carts, etc, as they are usually included with a used boat, and none of that stuff is cheap if you have to buy it new.
If the mast or sail is messed up, be aware that replacements are very expensive (thousands of dollars). Things like ropes and little stuff can all be replaced, Mirage drives are completely repairable. It's not a show stopper if the boat has been left out in the sun and the cloth mesh pockets are all faded and stretched out, they can easily be replaced for around $30 bucks apiece. But a very faded out boat means it has been stored in the sun too much, and I would be very cautious. Most of us do not store our boats in the sun.
Hope this helps you