Joists 2' apart is a real special challenge for any flooring cover let alone breakable tiles of ceramic or travertine. There is much more involved than just putting down a cbu (which adds nothing to the floor's strength), most of which is structural in nature. Your customers are right to be stressing; they probably know more about their floor than you may and realize there is no 'cbu-solution' out there, so they know they'll need to fork out for a fix. And seeing you put down cbu won't calm their fears, I'm afraid.
But their main problem is having gone out and picked up some travertines just expecting the world to conform to their decision...doesn't work that way and they just learned a valuable life lesson. I hope.
I'm surprised that any Building Code allowed 2' joists to exist under a bathroom, with live and dead loads being so variable. Well, perhaps this was a 'DIY special' - either way it's not a suitable substrate for tiling.