[wrxtasy]

They regularly use USB keys for movies and such.
Their TV is 1080p and has no audio out (ridiculous) and they want to use their Bose sound bar thing. It outputs mini stereo and optical digital. It's not actually a deal breaker if the bar can't be used.
Their budget is under 75 USD.
Has to be easy to use but can be harder to set up since I'll be helping them with that.

There are so many options, I'm just getting lost!

Thanks in advance.

Just get a USD$69 Xiaomi Mi Box as it meets all your requirements and is simple to use with an excellent Wireless remote.

http://www.mi.com/en/mibox/

The Mi Box has a built in Mini Toslink/Optical out port.

For Audio with your system and the Mi Box you buy a Mini Toslink cable, such as this one:
FosPower (3 Feet) 24K Gold Plated Toslink to Mini Toslink Digital Optical S/PDIF Audio Cable with Metal Connectors & Strain-Relief PVC Jacket

Post #1 of the Mi Box thread has links for where to purchase the Box.