Wireless Performance in a 100 year old house

The best thing to do is purchase a range extender, configure it to the access point downstairs and then bring the range extender upstairs even one flight up where the signal will be stronger.

examples of range extenders
ref link:  https://www.google.com/search?client=firefox-b-1-d&q=netgear+range+extender


Also verify if the user's ISP has issued him their latest wireless access point.  Alot of these ISPs have issued newer ones to their customers with faster speed and stronger signals

Old houses are often seen with plaster walls or ceilings.  These are beautiful and historic but the plaster is often held on by expanded metal sheeting on lath.  The metal sheeting functions very effectively as a Faraday cage, blocking most if not all of the signal through it.  The same is true for pressed tin ceilings.

In this situation you probably don't want to run a cable upstairs (though that would be the best solution) to feed a second access point.  You might look at powerline networking (HomePlug) as a network expander or a relay to an upstairs access point.  When it works, it works very well; when it doesn't work, it doesn't work at all.

if the house that old has grounded outlets the powerline networking should work, but a house that old unless it was rewired probably does not have grounded outlets. I otherwise would recommend going that route as well.