The hard part may be taking the old barrel off if the rifle was neglected too much. I have removed 3 Rem 700ML barrels with no trouble but they were in good shape.
Is he using a scope on the rifle? A used barrel may not index the same way as the old barrel which would cause the sight screw holes to be off. This can be fixed but you need the right equipment.
I know of 2 kits that would work, one sold by ArrowHead Sporting Goods, which I have not used; and one sold by derfhunter on this board, which I have used 3.
Bolt conversion kit ~$90, Breech plug~ $55, rebarrel and install bolt kit~ $100
I personally would not put a factory barrel back on, the custom barrels shoot much better in my opinion. The down side is the price, around $800 installed but you could shoot BlackHorn 209 or smokeless powder. Now that I’ve mentioned smokeless, the barrel and breech plug on the customs are designed for it, a factory Rem 700ML barrel replacement barrel and bolt conversion kit NO SMOKELESS.