Interior grout stained

I have a customer we service that has requested help addressing a grout issue in one of their bathrooms. It is a hard stone flooring with a tan grout. Around the toilet area the grout had become stained. He took hydrochloric acid and tried to clean the grout. It darkened the grout to a brown. He would like to know if there is anything that can be done. I told him I didn’t think there was anything that could be done at this point but that I would find out from some others that may know better than myself. Any suggestions?

How did the brown coloration look in contrast to the stone? He could do what he did to that portion to the rest so it will be consistent. Outside of that, if there is a fix short of a total restoration, it’s beyond me

The brown was ok. Not great. Might be the only real option for him other than a remodel. I told him it would have been better to have left well enough alone and called before using such a strong and unstable acid.