I see your "friend" is using RO water. I'm not a hydro guy by any means, but I have seen enough posts on here about RO water lacking trace nutrients. Does the GH 3 part have sufficient calcium and magnesium? Most of the hydro grows I have read about on here use CalMag (the product) as an additive. Just a thought.

It could be a ph problem in addition, or seperate of any deficiency. Do you know what the ph of the solution is? Do you know if it gradually climbs between res changes? How often is the res changed? What ppm are the nutes being run at?