Dave

The powdery grout could be a number of things....

1. Too wet a mix resulting in weak cement to water ratio....

2. Too dry not enough water to create a strong set.....

I think your biggest problem though is you have a wetroom type floor with no tanking of any sorts......

So even if he had used the GT1 admix water would have still ingressed through the joints.....

I think you have 2 options really.....

1. Pull the floor up and water proof it ( tank ).

2. Remove the existing grout and re-grout with epoxy grout........this will provide a water proof joint.......

let us know which way you go.......