Hi tcat007,
What's the best approach for getting her thinking that foam is OK?
Foam is a very generic term and there are different types of foam materials (polyfoam, memory foam, latex foam) and each of them have their own pros and cons as well as their own properties and "feel". The only way I know to help someone change their thinking about anything is to encourage them to to do the research so that they can learn the differences between different materials, hopefully test them out in person at local stores, and make informed choices based on the facts and the pros and cons of each different material.
Excluding all foam materials from consideration because one mattress design was too firm would be like deciding never to eat any fruit because you didn't like McIntosh apples.
Every material comes in a wide range of softness and firmness levels so softness or firmness has much less to do with the material itself and much more to do with the design of the mattress.
As an example, the Responda Flex you linked has 2" of memory foam over a support layer of firmer polyfoam (unfortunately they don't provide any information about the foam quality). The memory foam layer is quite thin which means that you would "go through" it more easily and feel more of the firmness of the support layer than you would if the memory foam layer was thicker and isolated you more from the firmness of the support layer below it.
The SleepEz is latex which is a completely different material than either the memory foam or the polyfoam in your Boyd mattress. I would also keep in mind that almost all mattresses you are likely to consider have some kind of foam in the upper layers which is the main part of what you feel on a mattress regardless of the support layer underneath it.
Any way to guess at the ILD of a 20 year old Beautyrest?
Unfortunately no. The only way to make an estimate of its softness would be your own subjective assessment of your mattress. ILD is only used to measure the softness/firmness level of a single layer not a complete mattress and the major manufacturers don't provide the ILD of the layers in their mattresses. Even if they did ... ILD is measured differently for polyfoam than for latex and ILD has little relevance at all with memory foam because it changes with heat and humidity and time on the mattress. If you wanted a very rough approximation you could test various local mattresses that were "rated" as being either soft, medium, or firm by the store that sold them (as subjective as that may be) and then compare how your mattress feels to you in comparison and approximately where in the "softness/firmness range" you think it may be.
Phoenix