There's no need to be obnoxious, you can just tell him that he should have used who instead of whom. If you wanted to be extra neighborly, you could have provid

There's no need to be obnoxious, you can just tell him that he should have used who instead of whom. If you wanted to be extra neighborly, you could have provided a cool rule of thumb for gauging when to use which, suggest as "Try substituting him or he; him corresponds with whom whereas he corresponds with who."