interest rates have also been in a long term secular decline, something that won't happen forever.
Over the medium to long term, bonds will be a tough play, if only because interest rates are so low and can't go much lower. Since bond prices and interest rates are inversely related, this is bad news for bond holders. BUT - current interest rates could stay low for a long time. The Fed promises no action until 2015, but if the economy remains poor this could be put off for many years.
So your friend is in some ways right. Their prices will have to go down at some point in the future - the problem is that no one knows whether that point is within the next few years, or within the next decade or two.