if the tile guy used plain old regular grout, it can be redone, but they'll have to scratch out about 1/2 of the old grout, and use the new stuff on top of it. if he tries to do just one spot, the grout color may not match up w/the older grout.
I wouldn't have used sanded grout unless the joints were larger than 1/8".
The Mapei FlexColor CQ doesn't need to be sealed. it's a urethane based grout. but if he used the regular grout, it's cement based and does need sealing.
after it's all finished, and one color again, I'd call in a marble restoration expert. they can re-polish the stone, or hone it for you. then have them apply that Tenax sealer. that stuff is bulletproof w/water absorption.
Remember, no harsh cleaners. I use this for my marble.
you can also try this for a stone/grout sealer. can't believe he didn't seal everything for you.
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Marble shower pan surround
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