Even those who agree that federal minimum sentences is a usurpation of the proper role of judges should also understand that a law is a law. If the judges want to rule the guidelines unconstitutional, that is an approach they could take.
But simply ignoring the law is not something a judge is empowered to do. And a member of congress, responsible for passing the laws which govern our country, has every right to send letters to people who ignore the law.
That said, I think the appropriate action, if this was because of his position in a committee, would be to get a vote in committee to send the note.
If he was sending it simply because he was the congressman of the district where the judge resides, that would be fine but he shouldn't say it's because he's the chairman of the committee without having the committee backing.
Actually Nullification (be it by a Jury, a Judge, or an enforcement official), while unspoken of in society is one of the more effective ways to combat an overreaching Legislature.
Point well stated.