Hi matthewsoft,
The better options I’m aware of in the Chicago region are listed in posts #2 and #4 here which includes two sister companies (My Green Mattress and Quality Sleep) which are both members of this site and are probably the first place I would visit. They make a wide range of mattresses including all latex.
The Sealy Cedar Point has 3" of 4 lb memory foam which is medium quality over a 7" polycore which I believe is 1.5 lb density. I would tend to avoid both of these materials in the weight range that your friend is in and foam softening and breakdown will probably lead to the relatively rapid loss of comfort and support. A forum search on “Cedar Point” (you can just click this) will bring up a few more comments about it.
He may be able to extend the life of the mattress by adding a topper which will absorb some of the compression forces of sleeping on the mattress and may extend the life of the mattress but with 3" of softer memory foam already in the mattress thicker layers of softer foam on top of the mattress may also have a negative effect on support and alignment and can affect the response of the memory foam underneath it so the results of a topper may be somewhat unpredictable in terms of PPP. A fairly thick mattress pad may also have some effect on improving durability for the same reason (and the “feel” of the mattress as well) but with weights that high he will also “go through” the topper or a mattress pad into the memory foam comfort layer below so it may do more to extend the life of the support core underneath the memory foam than the memory foam itself and if the memory foam softens then it would be time to replace the mattress anyway because a topper will still “follow” the soft spot in the layers underneath it.
Whether it’s worth adding a topper would depend on how well the mattress “fits” his needs and preferences and if there is any “room” for adding further softness and the potential alignment risk of adding more thickness and softness to the comfort layers of the mattress.
Phoenix