If the government is able to trace that your coins were spent by your friend, you can be sure they can also beat your identity out of him.
It seems like it would at least put up another barrier in the chain of evidence. Also, if the trade is done on this forum (i.e. not by meatspace friends), then identity would be pretty hard to determine.
And the swap only requires trust in one direction. If I trust Member A on this forum but A doesn't really know me, I could simply send him my wallet first. Once he's moved the coins into a fresh wallet, he would send me his original wallet. From A's perspective, there's nothing to lose.