Our tiler put Grout Boost on after laying grout, thinking it was grout sealer. What should we do?
Grout Boost is meant to be mixed with the grout before installation.  If you can clean it off and the grout looks ok afterwards, I imagine you could use regular grout sealer. 

Personally, if you are paying them to do the job, I'd want them to dig out the existing grout and re-grout it.  There is no need to tear up all the tiles, just the grout. 

You might try emailing the folks at groutboost.com and see what they recommend specifically.