Wow, this is actually pretty cool. I've never seen this method before. I would think it IS safe, as long as the polystyrene has had plenty of time to off gas before putting your frogs in. And of course cleaned well. I have styrene in my vivarium, purchased from exo-terra.
By the way, Kevinp, polystyrene is actually a type of of plastic, not grout... it is used to make many different types of products, including styrofoam. As far is it being and endocrine disrupter, that is true. I think it is still debatable if it will effect our frog(s). Wouldn't it need to be in the water, and the frog soaking in it for any to absorbed? I don't know, but it's an interesting conversation to have for sure.