Files/Folders on my external hard drive are suddenly not recognisable, please help

Did you try Diskutility.
Verify Disk should tell you if something in the file system is damaged.

What file system is/was on there? With HFS the only thing the PC could have done is reformat in which case you need some data recovery software and hope he didn't write over anything.

exFat or NTFS could be broken but it is odd.

ink files are Windows files. They don't just appear for the fun of it by simply using the drive. All the files shouldn't be readable by OSX because they aren't OSX files.
Not all is lost unless he wrote to much data no it.
https://www.google.com/search?q=dat...pw.r_qf.&fp=a1b45d574b2d2389&biw=1120&bih=634
Recovery software can find most of the old files again if he only reformatted.

 

I would try to figure out what your brother did exactly.
He definitely did more than just plug it in when there are lots of files.
Data Recovery is a hassle as you will soon find out it is a matter of last resort. If you know exactly what he did there might be an easier fix.