It depends on your individual state residential real estate regulations.
North Carolina, where I've been a real estate broker for more than 22 years, is a "full disclosure" state. The seller and their agent MUST disclose all known defects, malfunctions, and anything else that will affect the value of the property.
Here in NC, there is a residential property disclosure statement (signed prior to a contract by both sellers and buyers) that specifically includes the seller disclosing the type of plumbing pipes.
However, there is no law anywhere that I've ever heard of requiring a seller to replace anything. They just have to disclose what is there and whether it is functioning as intended or not. A residence only has to meet the building code of the year in which it was built...in your friend's case, 85 years ago.
While obsolete galvanized pipes are notorious for scaling up on the inside until eventually becoming completely plugged, and the usual life span of them is supposedly about 40 years, there obviously are such pipes that have lasted twice that long and more.
As long as they aren't leaking and he disclosed them, he should have no problems. If he replaces them, he likely won't get his money back in the sale price, especially in this market.