Hmmm....yes and no? Combining a couple sources here; It's necessary to treat K and Mg as important as Ca but Ca needs to be strong throughout the entire project for them (K/Mg) to respond and perform as well as they can. Ca is for sustaining fertility. If you have a huge canvas of Ca, you can roll out K in spades (and it will be utilized way quicker.) Mg being the catalyst between them and other elements. It does a bunch on its own with less- but if you let it take too many of the spots where Ca should be, it's incredibly hard to knock off.
"Magnesium is only 24 parts in the total of 900 or more parts o± in either of the two kinds of chlorophyll." But without magnesium- those hemes are completely useless.
Yes- the ratio of Ca to K would be lower during moments like those- but as a function of raising the K and leaving the Ca to operate on high.
And yes- they both help with cells/walls but they both contribute to strength/turbidity/what have you. K and Na are constantly moving in and out of cells and as such, without K, transfer pumps would not displace waste or communicate other elements/proteins between cells. Calcium is doing much of the signalling of these events (purportedly.) Without Ca to direct traffic- the whole thing is at a standstill.
I think your buddy is 95% right but for just minutely different reasons/wording.
