I do not know [definitively] why he would "strongly advise against taking vitamin C if hair analysis shows 4 low electrolytes" but it may have to do with the fact that vitamin C is a good chelator of minerals when they need to be excreted., e.g. mercury, aluminum, lead, etc., etc. However, it also acts as a promoter of mineral absorption when they need to be absorbed.
Perhaps his fears have to do with the fact that, if a person continuously exceeded their Bowel Tolerance limit, i.e. thereby promoting too much [benign] diarrhea, they would, at least theoretically, have the potential of losing too many minerals in their "watery" discharge/stool.
My personal experience in this matter may be of some use/interest: I have ingested well in excess of 50+ grams of ascorbic acid every day for the past 20 years [that's over 900 pounds worth], and my health [at age 64] is what I would assume most people would call excellent, e.g. no sickness, doctors or drugs. If "mega-dosing" on vitamin C was harmful to one's [overall] health, surely I [of all people] would have a problem by now. But I can assure you, the exact opposite has been and is the case!
I hope this helps [as just "grist for your mill"], Pynegi.