Wow - that's an oldie ... prob only about 50% efficient.
Looks like 4 burners, totaling 100,000 BTUs.
The burners can be removed by grabbing each, and pushing forward, out of its gas inlet nipple on the manifold, then lifting up and out.
This alone may facilitate access to the thermocouple.
.. and, do the lady a favor - while the burners are out, give them a good cleaning, Wash them, then blow them out with compressed air. And vacuum out all that accumulated loose crud.
When you re-install the burners and fire up the furnace, adjust those litte air intakes you see in the photo, for optimum flame pattern ... just a hint of yellow at the flame tips.
If you want to be _really_ nice, slide the blower assembly out - remove and wash the squirrel cage fan, and oil the motor ... probably hasn't been done in years, if at all.
Joe