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Please note that you do not need a Roku device to play movies off a USB thumb drive. A Roku streaming device lets you watch entertainment—like TV shows, movies, YouTube videos, and more—from the internet on your TV. Just plug it in and connect it to your Wi-Fi network. On your screen, you'll have instant access to streaming services like Netflix, Prime Video, and ITV Hub, plus you'll get access to 100,000+ movies and TV episodes across thousands of free and paid channels. You can check here to learn more about how Roku works and here to help you decide which Roku device suits you best.
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@kaitlin4599, Roku devices do not support AVIs. They support a limited number of formats which you can see here. The Roku 3 was a good device in its day, but they're getting pretty old. You may still get some use out of one. I wouldn't bother with anything older than a Roku 3. You can see all the models on the Wikipedia page.
As @RokuEuniceL implied, Roku devices are mainly for Internet streaming and their usefulness for your own media is limited at best and definitely not a strength of Roku devices. The only current models that have a USB port are the Ultra (4802) and the Streambars which are even more expensive than the Ultra. Roku reuses names and I believe all "Ultras" except the Walmart exclusive "Ultra LT" had a USB port. When it comes to Roku devices, the model number is more meaningful than the name.
Has your friend inspected his TV and other devices carefully? For example, my 2008 TV and 2011 Blu-Ray player can both play movies from USB drives.
If you ever need to update your Roku device for your friend, you can consider purchasing the new Roku Express (3960), or the Roku Express 4k/4k+ (3940/3941).
The above are $30-40, with sales running at Roku and stores like Walmart and Best Buy all the time.
You can then purchase an Ethernet adapter with a powered OTG USB hub that will allow the capability for both Ethernet access (if desired) and the ability to use USB storage drives/thumbs drives with the Roku device. ($13 example - https://www.amazon.com/Ethernet-Replacement-Streaming-External-Internet/dp/B09BFPQ3V5/ref=mp_s_a_1_1...)
Another method would be to use any Roku device on the TV, and then look to see if your router has a USB port. (Either 2.0 or 3.0 will work). May be able to just enable UPnP or DLNA mode/access in the router settings and then play the movies through the Roku Media Player. (As mentioned by others, your movies will have to be a compatible format for Roku Media Player to see and play them.
Just a couple ideas for you. Some find a Roku device with a built-in USB to be easiest, but there are ways to get this feature with the less expensive models.
As for supported formats, here is Roku support article with that info. https://support.roku.com/article/208754908