Not much helpful info ... is this a day trip? Or will they be in DC for a few days, staying at a hotel? There are many really nice hotels, but a number of ‘fleabags’ too, and those are not well located.
If they stay close to the Mall and tourist areas, they should be fine. The monuments themselves aren’t AC’d, most are just outdoors.
I imagine all the Smithsonian buildings that line the Mall have had their AC restored. The Capitol and various House/Senate office buildings; the SCOTUS bldg and Library of Congress should also have their AC restored.
Assuming they don’t have respiratory problems, and if they carry and drink lots of water and use sunscreen, wear shade hats, and normal heat/sun protections, they should be all right. Not comfortable, but fine.
Thank you