Whole house water filtration-any advice?

[replicant_argent]

It really depends on what he is "filtering." Whether it is particulate or chemical imbalance he is trying to correct. Softening is one thing in whole house quantities, filtration is another. Whole house carbon filters, for instance, can (when sized properly) be very expensive to maintain, and if not maintained, a complete waste of money. (Not to mention very rarely needed) There is a lot of voodoo science out there, and even in the most established companies, "overeager" selling and bullsh!t. Treatment does not necessarily = filtration. If he has high THMs, herbi/pesticides, arsenic or some other nasty, his issues may be far greater than writing a check.
On the other hand, putting in a really nice RO system could cost 3-4 grand for drinking/cooking water in a large home, and be completely feasible and reasonable.