You can't, although you can partially decompile them with the Papyrus Assembler. I don't think that's documented on the wiki, though. I'll see if I can find the information you'll need for that...
If you need to edit these scripts, you'll need to ask your friend to send you the source files instead.
Cipscis
EDIT:
Here you go:
To decompile don't include the .pex extension. To decompile
scriptname.pex you invoke this:
PapyrusAssembler scriptname -D
Cipscis