Hello and welcome!
Why can't friends and family just do what is asked of them- nothing more, nothing less!
How frustrating for you!
I imagine if the conditions are bad enough to kill a pleco, your shrimp will fare worse. You'd be better of doing an aggressive gravel vac and manually removing the slime or fungus or whatever it is.
What are your numbers for ammonia, nitrite, and nitrate?
My best guess is that your friend over fed and didn't do any pwc
so the ammonia spiked.
Lots of large water changes using prime or an equivalent quality conditioner will lower the toxicity of the water and help balance the ph again. It's strange that the ph went UP instead of DOWN. Unless it was raised purposely to correct a drop in ph.
I hope the rest of the inhabitants pull through. Keep us posted