He can't. Cinavia is embedded deeply inside audio stream, so even if you repeatedly use lossy conversion such as MP3, Vorbis, AAC, etc it is still there. Media server doesn't work either.
No one has found a way to remove Cinavia. Some have found some ways to bypass it, but bypass is not universal. For example, in PS3 you can intentionally set the time backward to get more playtime (by default Cinavia on PS3 kicks in 20 minutes). In some earlier Blu-ray players you can use DVDFab which will add some dummy files to trick them, but modern players don't fall for that trick.
Unlike AACS and CSS, which were poorly designed, Cinavia actually doesn't have any big loophole at the moment.
Which is why I say you either have to remove the audio portion, or use an old Blu-ray player. Cinavia was introduced in 2010 and mandatory in 2012, use that as a guidance to find a player.