I would mention that older homes were balloon framed, meaning that the wall studs went from the foundation to the roof, with the floors and ceilings on ledgers nailed to the studs, and that this creates a fire run for fire from the bottom to the top, and that especially for two story homes, this creates a chimney like effect and drafts upward quickly.
Then I would state that modern building methods found that fire stopping was necessary to reduce the risk of, and damage from, fires, and that most homes built today are platform framed where there the the foundation, the first floor platform, the first floor wall studs, then the second floor platform (or top plate and roof structure), all of which break off the stud cavities at every floor and ceiling.
I would follow that with 'And there is no practical way to go back and add fire stops to an older home which does not have fire stops, especially balloon framed homes.
You could then add that if the old knob and tube wiring were replaced with modern wiring, they could loose fill or foam fill the stud cavities with thermal insulation, which would not only help reduce energy costs, but also help stop the spread of fires and the drafts those fires create.
I know you know what balloon framing and platform framing is, but you would need to explain it to your client, so I included a brief description above also.
I.e., 'You are buying a fire trap, there is no practical way to fix it, but, not to worry, your fire trap is no worse than the fire trap of the same relative age next door or across town.'
