Mercedes 300 E won't start

[Mercedes300E]

I have a friend that purchased a 1987 Mercedes 300 E. She is having starting problems. She has had the car in to be checked out but, they cannot find anything wrong.
The 1st time the car wouldn't start, she had a mechanic come he thought he'd have to push it to start it but out of the blue the car started right up. Yesterday when she was leaving work to go home she started the car and got nothing. She said, no noise of the car turning over. The lights would turn on when she applied the switch but then of course go out when she turn switch off.
Please if anyone has any suggestions I would appreciate feed back