Am I the only one who did not know this existed?
http://devtools.web.roku.com/RokuRemote/electron.html
It is an AppImage so I recommend AppImageLauncher to integrate it with your system if you are interested
sudo add-apt-repository ppa:appimagelauncher-team/stable
sudo apt update
sudo apt install appimagelauncher
Interesting. I just got my mum a Roku so I'll try this out soon. Although she seems to often prefer having the Zorin Lite laptop plugged into the TV and using IPTVnator with playlists like IPTV-org, even though there's no remote ability (yet. Installing PC remote control app on her phone is another thing I need to look into.)
It has worked flawlessly for me so far. I tend to work with a tv going in the background for background noise so it is useful to me to be able to control it without moving my controller all over the room.
I used to have a similar setup for my parents. We had a very long HDMI cable from their tv to their laptop and they controlled the laptop which mirrored to the tv.
Depending on the situation a wireless mouse and keyboard might work well. But if they cant read the text on the TV well this may not be a good solution
Re text size, there are things with a "10-foot interface" to make that easier. Kodi is one, and it sounds great in theory (and I do have it installed on my mum's Zorin laptop). But in practice I need only about 60 seconds of actually trying to deal with its UI before it drives me up the wall and I want to kill it with fire!
(But other people love Kodi. I guess for me it just breaks too many of the conventions I've learned over the last 20 years on how to interact with a computer or media player, and the most basic task quickly becomes a nightmare.)
PS Setting up IPTVnator for my mum and hanging out with her while she watches, I've now seen so much NCIS: New Orleans that I get the reference of your user name 
I've given the Roku dev remote control app a quick try. It's nice to have the option of running from the PC, although so far I haven't seen a compelling use case for it over using the phone app. At least for my mum's current set up. I might need to experiment more to see the benefits (e.g. explore the scripting possibilities?)
I also found another app, Controku.
But I can't find a Linux binary on its homepage nor in Synaptic (and I've no experiencing of compiling from source or "gitting" software).
There's a Reddit thread where the developer answers questions about the app, but the current Reddit strike means it's Private Viewing only.
Controku: Control Roku devices from your Linux desktop (or phone!) : r/linux
I have seen Controku before but it just is not as seamless when I messed around with it a a year or to ago.
I have not messed around with the scripting on Roku Remote Tool at all. Might can do some interesting things with it
