If it is a deep well it is hard to understand where high nitrate is coming from.
If it is a shallow well, there there may be infiltration from yard runoff (fertilizer) or if it is near a septic system it could be caused by contamination from that. If it is on a farm it could be contaminated from a manure pile.
A shallow dug well with a crock or cistern could be contaminated by some animal having fallen into it that is decomposing.
It may be that the well head is not properly protected or sealed. If the well is not properly protected then that should be fixed, the water pumped out, and retested in a couple of weeks.
If it is a shallow well the solution is probably a deep well, but then you have to consider the possibility of hard water or even arsenic.
He should have a sanitary survey done and try to find the cause. There may be a local health agency that would help with that. I do a lot of work in Maine and there is an organization called Maine Rural Water Association that helps people with that kind of problem.
After the sanitary survey is completed, I would have it retested.
Putting in an RO sustem for household water would be my last choice, even after considering a good deep well.