Leaves full of brown spots

Yes a cal/mag problem, but one dose will not fix it and recovery takes many, many days as it is slow acting and the plant must actually process the cal/mag before it can be of any benefit. The damaged leaves will not recover, but the additional cal/mag will slow down the progression soon enough. I do not know what cal/mag your friend is using, but a dose of full strength as recommended by the manufacturer and given once a week would be appropriate. Depending on how close you are to harvest, if you are really close, there is probably no point in doing anything, as recovery takes at least 2-3 weeks, if not longer. Remember too, what is showing in the leaves now, is a reflection of what occurred in the soil 2 weeks ago, damage is a slow process, just like recovery will be. Next grow, start using cal/mag from about the third week of flowering as a preventative, which is always better than any cure! Hope this helps, Organoman.