Ask your friend to check what format he is sending the attachments in.
When creating the mail in Outlook, go to
File->Properties->Send Options and which of the attachment format options is set.
Usual choices are Don't Know, Mime, Uuencode, and BinHex
with Don't Know being the default. Suggest you ask him to do a test sending the same test file with different settings, see if you get one that works.
Outllok does have some known problems with certain mail formats/ISP combinations