I found a lot of great music at etree.org, but I haven't found any friends there. The procedure I outlined above was for music from "friends,etc".
Retro, have a look at the import menu, just to verify that FLAC is enabled. Also, what is the extension of the friendly files-- .flac or .fla? I believe older systems will truncate the extension, and maybe your friends' computers need upgrading.
Failing all else, this sounds like an UNCONFIRMED bug reported about six months back. Out of the tens of millions of MC users only one reported it. Now that I think of it, maybe it was your friend.
At any rate, since then the entire FLAC decoding and encoding process has been upgraded, a huge undertaking for which jriver spared no expense (although kudos are due to ghammer for spearheading the effort). So I doubt that the two are related.
Maybe Alex B has some further info on this, although I think he's finnished.