Determining unknown filesystems from OSX

Years ago I set up a Linux-based multimedia PC for my parents to store all their music and movies on, as that’s the OS I use on my home computers and so if/when things went pear-shaped I’d be able to guide them through fixing it over the phone, as they live overseas. Recently that computer died of a hardware failure, and they’d like to access their library of multimedia from their MacBook.

The problem is that the filesystem on the multimedia drive was set up as a Linux one (probably BTRFS or ext4), and I don’t remember what I formatted it as. They have a SATA-to-USB bay that they can plug the multimedia drive into, but OS X doesn’t recognise the filesystem and we don’t want it to attempt to “repair” or reformat the drive and risk losing everything (the backup drive also used a Linux filesystem). I’m unable to visit them and fix it myself, and at any rate I’m not sure what tools there are to use, being totally unfamiliar with what’s available for OS X.

How would they go about determining the filesystem on the drive, and/or mounting it from their MacBook?