Your 'friends' plants look generally very healthy so I doubt it's over watering or root rot in this case. Looked like a deficiency of some sort so I did a couple minutes research. Sounds like coco has a tendency to absorb calcium. Apparently you need to supplement. I'd recommend trying that cal-mag stuff and check the pH of the run off just to be sure. Get them back on their normal strength feed too, otherwise you'll probably compound the problem. You're better off with a tiny bit of tip burn than under feeding from my experience.
Is using Coco the new fad? Better off sticking with rock solid proven mediums like a perlite/vermiculite combination and normal nutes if you ask me.

Nice serated leaf edges. What's the strain?

See links I found re: coco and calcium/mg.

What's the issue with calcium/magnesium? - International Cannagraphic Magazine Forums

CN: Cannabis Community > epsom salts,canna coco nutes,coco perlite..hhhmmmm

First Coco Grow Going Downhill - Grasscity.com Forums
CFLhydro Reviewed by CFLhydro on . yellowing at tips then drying up hello im trying to help a freind out whos having problems growing. hes growing in coco soil using coco a and b, hes growing in a tent with 2 400w lamps with the temp at 26C with the lights on 18hrs a day, he used 30ml of coco a and b and 5 ml of bio - stimulant and and the plant started to yellow at the tips so hes been flushing it out for the last 2 weeks with ph water at 6.0 the new shoots are coming green then still yellowing at the tips some curling up then drying n falling of. can anyone Rating: 5