Should I replace w212 wagon with self leveling rear air suspension with coil springs or try and fix the air suspension?

Hello, a friend has a w212 wagon e class with just the rear air suspension. Yesterday it finally failed and wouldn't rise back up, he's thinking about doing a coil spring conversion with a kit designed for the car. What are you guys opinion about this, the engine and gear box is in excellent condition so is the whole car and he loves it. It has 223k miles on clock but you would think its a 50k mile car. He wants to keep it running and make it high mileage hero.