Grout falling out

I have a customer that grouted the bath tub surround and within days the grout started falling out in big pieces. She is pretty upset because her husband is upset with her for not sealing the grout before using the shower the next day. I reassured her that wasn't the reason why and told her it was due to movement. She doesn't know how it was installed because they hired someone (a very old man according to her) to do the work. Is movement the only cause or could it be something else also? Everything was bought from us at HD.

Typically "movement" is the first cause of those type of grout issues. The very fact that grout when installed sorta locks itself into the cavaties and stays in place is what would make it hard to swallow that it is simply falling out in a matter of days. Just falling out really isn't all that possible.

What brand/type of grout are we talking about? Did they use Polyblend.

Grout improperly mixed by using too much water will ruin grout and cause it to erode when the showwer is being used. This will happen immediately or within days.

Have you seen these "big pieces"?

You are correct that not sealing the grout has absolutely nothing to do with the grout falling out so soon. I can assure you this is going to be installer error if Polyblend (or any Portland grout) was used.