+1 HE-500 is great for nearly everything IMO. The bass performance may fall slightly short of HE-400, but is still far better than the bass of most dynamics, including K701 and HD650. The mids are simply gorgeous on HE-500 and even better than those on HD650. The highs are much better than HD650 or K701 - much cleaner and faster, and with great sparkle without HE-400's overemphasis.
He can resell it easily if he won't like it, as HE-500 is very popular. Not everyone likes HE-500, but most people do I think, so it's a pretty safe bet.
It's arguable really that the HE500's bass is inferior to HE400s in every strict sense. The 400's bass is tighter (one of the most tactile clean basses I've heard, right beside LCD2 / HE6), but 500's bass extends quite a bit deeper and has significantly more rumble, it also at least to my ears is a tad more "tonal" (easier to distinguish one bass freq from the next) than 400's bass, perhaps BECAUSE it is not so incredibly tactile.