Slate bathroom floor for a friend.

[oldhouse]

Day before yesterday, a friend called me over to ask about his plan to tile his floor in a bathroom addition with slate tiles that he had been waiting to use. So, I looked it all over, and now I have some questions.
His addition is on a concrete slab, but the bathroom is stepped up 6". He layed 2x6's every 6" and then put down a subfloor of 1/2" exterior rated OSB.
My response to this was that he needed another 1/2" of subflooring, then either ditra or CBU. But, I just wanted to make sure you all agree with that.
Thanks

[bbcamp]

Yes, he needs another layer of plywood, then an underlayment. Since his sleepers (joists) are on 6" centers, he could use 3/8" plywood.

Now if that 6" is a typo and should really be 16", then use 5/8" plywood. Stone needs between 1-1/8 and 1-1/4" of plywood/OSB in 2 layers.

[dgunnels]

Does he really want to use slate in such a wet area?

[oldhouse]

Yes, it reallyl is 6" on center. And, yes he really wants to put slate in a bathroom- well, his wife wants it, so you know what that means. It is a 'guest' bath so likely won't get heavily used.
Any other bits of advice?