Understandable, but not possible via official means currently, unless you go the AirPlay route. My initial post is all the official methods available without using Airplay (the path Tony is explaining)

One alternate path would be if they own a second generation (720p) Apple TV and are willing to go the jailbreak route. That opens the possibility to get a DLNA client on the device. This won't work with 3rd generation (1080p) units though, no jailbreak exist for them, and people have been trying for years now. One polished route for this is FireCore. I used this on the old first gen AppleTV, and it was well worth the small price.

A different alternate path is PlexConnect. (Thanks Tony for the Plex reminder) . There is hints PlexConnect can now run on a Synology NAS. Basic concept, it acts as a proxy to the AppleTV and is able to inject Plex content into the standard AppleTV software. I haven't had time to try this out myself, but it looks promising. It is a hacky way to do it, will likely be fragile, and needs updating from time to time.