In the long run it will pay off. Plus the upper apartments will be cooler. The walls most likely need insulated also. Depending on what he is expecting for his ROI (Return Of Investment), all depends on who he rents to and how much he charges.
The one thing about getting the roof torn off if it has been on for a long time. Any rotting wood can be addressed and also air sealing can be down around fixture boxes and wiring. Same that any critters that have found their way in that space, can be kicked out.
The good thing about getting up in the ceiling, will be that he can install wired CO/Smoke alarms, if they are not already installed. Of course once he starts opening things up. he can be required to do certain upgrades. Denver now requires Sprinkler systems in apartments. As for if they are required in existing structures and with this work. Depends on the municipality code office.
