Recumbent Accessories
There's very little you need on a 'bent trike that you wouldn't also need on a conventional bike. Rear jersey pockets are useless on a 'bent, of course, so it's nice to have an accessible place to store energy bars and stuff like that (I've used a bag that's intended to fit above the top tube, behind the stem of a conventional bike, and a small fanny pack turned sideways). Rearview mirrors are a necessity, but I think all new trikes come with them. Many trikes lack good bottle-cage mounting points, requiring you to improvise with clamp-on cage mounts; I use a Camelbak Unbottle and some webbing straps.

Trikes often have oddball tire sizes, which you would need to know if you were going to get her spare tubes. Normal seat-bags won't fit, and some trikes don't really need them. There are recumbent-specific bike shops on the Internet. It wouldn't hurt to ask her what she got.

So, regular stuff: a cycling multi-tool is always handy to have (assuming you know how to use it). Likewise the typical frame pump, patch kit, lights (may need special mounting adapters), etc.