Hi DiveCat,
If your mattress has no soft spots or dips and all it needs is some extra softness and pressure relief then it would be a good candidate for a topper.
The topper guidelines in post #2 here and the posts in links to can help you make the most suitable choice.
I would also keep in mind that a topper will reduce the ability of heat to reach the memory foam underneath it which means it will be firmer than it would if you are sleeping on it directly so you may need to “compensate” for this by choosing a little thicker than you otherwise would.
In most cases … “just enough” in terms of thickness and softness is the leas risky choice in terms of negatively affecting support and alignment on your mattress and exchanging one issue with another.
Given that an average thickness for a topper on a mattress that already has some softness in it would be “in the range” of 2" or so … a 3" topper (to offset the firmer memory foam) may work well for you unless the surface of your mattress already has comfort layers that are already thick and soft.
Both 19 ILD and 24 ILD are both in the soft range but you may do better with the lower ILD and he may do better with the higher ILD. If I was in doubt … and choosing a slightly thicker topper … I would probably tend towards a compromise that was a little firmer than softer because it’s less “risky” in terms of alignment. You may be able to choose a split topper for each side as well and then have them glued together in a single cover if a manufacturer was able or willing to do this for you.
Some of the better sources I know of are listed in post #4 here (also linked in the topper posts).
Phoenix