If you can access the wall cavity, adding rock wool (mineral wool) insulation will make a big impact by reducing vibrations and sound transfer. 

R19 being 5.5" won't fill the cavity, and the 9.25-10" that R30 comes in would mean that it would have to be compressed a bit.

They just need to find a hole that goes into a cavity. 

We ended up just doing a shiplap ceiling to eliminate having to drywall and filled the cavity between the two ceiling with rockwool and it has made a pretty substantial difference that wasn't crazy expensive and was a two day project due to eliminating the need to finish drywall.  

They've mentioned they have asthma and allergies and that's why they're upset about have to live in a place that's being repaired, so I've offered to both hold off on the repairs and reduce rent to make up for the condition of the property, I've offered to have a contractor just rough in the drywall so that the studs aren't visible and to encapsulate everything, even though everything in the cavity was removed during the remediation, but they now aren't responding to my texts whatsoever.

When you get the 2 pm call that the kitchen sink drain is leaking and you are filling a cavity in someone's molar, you can't exactly get up and leave to take care of the sink. 

You could then see into the ceiling/floor cavity to investigate where it's coming from, if anywhere. 

Imagine our surprise to find "Great Stuff" sprayed everywhere, including inside the outlet and light switch cavities of interior walls!

If it's still moist inside when you open up the wall be sure to put fans on it to dry the cavities out thoroughly.

To make a long, expensive story short, we discovered that builder failed to insulate an entire 3rd attic space and didn't seal wall cavities up in the main attic or insulate chases properly (Inspector failed too).