Title, basically. The bathroom sink was a bit clogged and she used a drain cleaner to try to clear it. She added hot water, and it seems to have turned to solid cement. Water is not draining through it at all and I can't even chip away at it.
I'm mildly impressed at how fucked it is. Just wondering if anybody has come across this before or has a handy solution. Otherwise it's looking like I'll have to pull the pipe and put in a new one.
Edit: update. One helpful commenter mentioned caustic soda, which helped me utilise Google more accurately. It looks like the wrong proportion of caustic soda was used, as the crystals were poured directly into the drain, whereas it should be dissolved in the appropriate ratios first. This means that there's a solid mass of caustic soda that has formed, which is extremely hard.
Recommendation is essentially physical removal. In theory, an acid might counter react, but this isn't advised because it could give off toxic gas, will only react with the top surface of the mass, and also can create a lot of heat that will damage the drain.
Thanks all. Link here in case a future person has the same issue.
https://www.hunker.com/13417422/how-to-clear-blocked-pipes-and-remove-solid-caustic-soda