if she's already on the google or amazon ecosystems, one of their devices might be the best choice for her, as no new account anywhere would be needed for the device itself. they'd probably be the easiest for her to set up herself.

rokuchannel isn't available on every device, so if she wants to watch anything that's "only" on it (roku originals, or current exclusives), she'll need a roku or something that roku supports. same goes for apple, but apple tv is more widely available--at least for viewing what's already bought.

note: if you want a roku device but don't want to pony-up a payment method right away or for only 'free' viewing, try creating one using a browser at https://my.roku.com/signup/nocc

if you don't want to end up being her go-to tech support, then choose a retail product used as it was sold.. not an ordinary pc or device that's been rooted or has sideloaded apps.

i do recommend a "device" vs a "smart tv". the device replaces the "smarts" and are easier to replace and upgrade. that allows you to keep the tv off the internet completely and use it as a big, dumb screen.