my brother took my car for a spin and he put premium unleaded instead of diesel now it wont run if i drain it and put diesel in will it be ok?

 

It sort of depends on how much gas was mixed with the Diesel Fuel and how long it was run like that.
I have seen 2 examples when I used to work in a Fuel Injection Shop.
On the one the Wife put Gas in the Tank and drove it until stopped. On that one the Pintel Tips on the Spray Nozzles of the Injectors were burned off.

In the other case the Guy realized he had put Gas in the tank and dumped several Quarts of Motor Oil into the Tank and drove the car to us to drain the Fuel Tank. In his case the Injectors suffered no damage.

In both cases there was no damage to the Fuel Injection Pump that was obvious when you ran the Engine. Perhaps in the first case the Fuel Injection Pump lost some years of longivity.

As far as putting Gas in the Tank that can happen to anyone.

The smart Guy was the one who dumped the Motor Oil into the Tank. That if yo let it sit a while woud mix with the Gasoline and restore allow some lubrication that Gasoline does not have.
Inline Fuel Injection Pumps take this sort of abuse better than a Standyne (Roosamaster)(Rotary type) Fuel Injection Pump on which a lot of Gas mixed with the Fuel and driven until it stops is likely to cause the Head Inside of the Fuel Injection Pump to seeze to the Rotor.

 

Drain and fill. You should be ok. It happened to my wife twice on her 1995 E300D.