I think it would make a very interesting exercise, but would take many hours! That's how so much of it is done in schools, but then they have several levels of staffing to produce these matrices for thousands of children each. To flesh out the "how to get from level A/KS1/Early level/Foundation to the next stage" there is a breakdown of all the requirements then how will we meet those. It eventually becomes a huge grid where you tick off that a child can count from 1-5, can order numbers from 1-5, can add within 1-5 using concrete objects for support, can subtract within 1-5 using concrete objects for support, can add using a number line, can add without using a number line, can subtract by counting back, can subtract by finding the difference, knows the terms "take away, minus and subtract", knows the symbol for minus, the symbol for equals, is able to write out an adding equation in the standard notation, etc. Then the same again for numbers to 10, to 20, to 100, to 1,000. And all that just to meet the eventual requirement at the end of 3 years "Child can add and subtract within 100 using a variety of methods, including formal written notation". It's then easy to show a parent why you have said the child is in this level and not that level becuase he still needs to get this skill and that skill, but that he has made progress from knowing none of those things to knowing 80% of those things, here are the specific gaps, when we have worked on those things he will be ready for this next step. I think it's hard for the parents to see any progress between grades, you are either at this point or you are not, they don't realise perhaps he is 90% of the way there.
Sometimes I think it's about explaining that a great foundation is time well spent, and that just because this one child is "ahead" right now doesn't mean that will last forever. Unless you are someone who does every single grade then you can explain there is a good chance that her son will skip a grade later, that perhaps this boy will do grade 1, grade 2, grade 3 then grade 4 and her son will do Step 1, grade 1, grade 3 grade 4, or whatever. Might be a good "in" for you to chat about practice habits too.
Perhaps "that boy" is strapped to his piano day and night.