Using the Theory hammer, of course they can. If you use any army correctly they have a damn good chance of beating another. I got beat lots when I had my last Tau army - and a few were against DE.
I honestly can't remember much, its just if your mate has been playing them for a while, he should look at the Tau codex and the DE codex, and see where he can make a hole. Just tell him to keep in mind that they are crap in combat, they have a not too bad armour save, but could be better, I would say that the Armour save is what he has got to pick at, thats what happened with me if I remembered.Can you remember what straegies they used?