Hi;
I probably would have lowered the pH some then chlorined it as I'd want to get maximum effect from my chlorine buck but that's for another time.

I owned a pool with a liner back in the late 1970's and we got our chlorine up there unintentionally a couple of times because of algae. I can't say how high it got but we seriously overchlorinated and we got lucky and the liner was just fine but then we may not have reached your level and it was an IG vinyl liner pool so maybe that's a difference too.

I wanted to ask if the BIL remember to feed the dogs and cats?
Jo