Hi Shopper,
Yes … this is polyfoam and would be very firm.
As you can see in the topper guidelines here … a 1" topper would be “just a touch to a little” bit softer. KTT has 1" layers of all the Latex International Talalay latex ILD’s and the softest would be 14 ILD blended Talalay. If you add another inch to your side then you may want to add an extra inch under the other side as well. I would probably add the inch to your side first and then if it works well for you add enough thickness under his side to make the height the same on both sides.
Phoenix