Maybe I'm thinking too simple, but could it just be that the glow plug system is not working? Easily checked with a voltmeter on each glow plug. Where is it located? If in a cold area, it probably won't start at all without glow plugging it. Ask the owner if the car had decent power when they brought it in and how well it started then. If it was low on power, then it the engine might be worn and low on compression, not letting it start.