Hi Hex92,
The first step would be to find out exactly what type of foam materials were in their mattress. There are many different types of foam (polyfoam, memory foam, latex foam) and each is very different. Every one of these types also has a very wide range of different versions and softness levels in each category and the specifics of the layering in a mattress (layer thickness and the specifics of each material) will also make a significant difference in the feel and performance of a mattress because every layer in a mattress will affect the feel and performance of every other layer to different degrees. Having said that … most of the initial “feel"of a mattress would be in the upper layers so these would be the layers that are the most important in terms of approximating the “feel” of a mattress. In other words if their mattress had 3” of 5 lb memory foam in the top layer and the foam underneath it was firm then if your mattress was as firm as their base layer then adding 3" of 5 lb memory foam would be somewhat of an approximation. If your mattress already has some soft layers on top (which most do) then you would need to take this into account in choosing the thickness of the topper or you could end up having too much soft foam in the top layers of your sleeping system and end up with a mattress that was too soft for you and having spinal alignment issues and discomfort or pain in your lower back.
Dn is also exactly correct. Foam density is the biggest factor in durability but every manufacturer has their own formula that can produce memory foam with very different properties. Post #9 here and post #8 here has more about the different types and properties of memory foam.
With latex (if that’s the type of foam that was in their mattress) the differences are much less if the type, blend, and density of the latex is the same but there are certainly differences between different types of latex and there is a wide range of firmness levels available in each type. There is more about the different types of latex in post #6 here.
Latex and memory foam are about as “opposite” as you can get (see post #2 here for some comparisons).
There are also many types of gel memory foams and you can read more about the different types of gel in post #2 here but in general you will find that it can have some effect on cooling for a little while when you are going to sleep and can also have an effect on the feel and performance of the memory foam but each type is different.
Similar density foams made in North America will have similar durability but they can still feel and perform very differently so it’s important to talk with each vendor to find out more about the properties of their foam and how they might compare to other types of memory foam of the same density. Some may have the type of feel and performance that you prefer while others may not although they will all share the slow response and other similarities that are common to all memory foams to different degrees.
Phoenix