Are the yellow leaves a warning?

Hey there Don't worry so much about the yellow leaf as you should about the intense green and the slightly burnt tips. It's probably a nitrogen excess. My advice: tell your friend to water only with water for the next two waterings, without Biobizz nutrients. If your friend can, have them adjust the pH. If not, they should buy bottled water from a brand that you can check the pH of online, so it’s between 6.2 and 6.5, and water with that.

I do agree with what's below. And, if I hadn't of checked your diary I'd of probably just said pull back on the N. But, you're using far too many products with N in them in the same time period. Yes, leafs can turn that way. But, by having very very dark green Background I do see your plant is been given to much nutes. And N is the most uptskable nutrient across varies stages of ph. Tis why its mainly the 1st we see in an overfeed. Pull back drop 1-2 nutrients. Or break them up. Give them on separate feed. 1 half at the start, the other at the end. And the LOW EC / SALT nutes can do in with the water feeds. There's just too many PH buffers in that pot. Your plant is struggling.

Hey there! It's normal that when the plant is in the second half of the flowering cycle (a few weeks since the first flowers are visible), some of the old leaves turn yellow. This is not a problem. Only when a lot of leaves turn yellow you should be worried, it means the plant has too little nitrogen. You could add a little fertilizer with nitrogen in it! Good luck with the rest of the grow, looks like your friend is taking good care of your plant!