Ereo is right, it would be considered mobile, yes. Fire OS is based on Android Open Source, in some ways it is almost a stripped down/customized version (though it is also considered its own OS) that Amazon runs their proprietary app store and other Amazon software on (Source: I have installed full Google Play store on Kindle Fires bought for friends). I'm guessing since there is no Fire OS listing on the wiki or the stardewvalley.net site, that Fire OS-based devices are simply lumped in with Android OS, which isn't super helpful/clear for folks that aren't aware, sorry about that. I had no idea Stardew was in the Amazon Store until you posted (it is usually pretty limited app-wise, nothing like the Android or iOS app stores), so I'm glad you did! I'll mention it to my friends.
Edit: Yep, search found where a forum moderator
confirmed Fire OS is lumped in with Android OS on official Stardew-related things. That's good for me to know for future, would have mentioned it to friends sooner.
Has she checked out the mobile controls page of the wiki for help? I fish with a paired a Bluetooth controller (iOS), so Ereo or someone who uses touch-based controls is probably a much better help.
The mobile version of the game has a number of different control options, all unique to mobile. These include tap to move using the device touchscreen and an on-screen joystick.
stardewvalleywiki.com