Damaged timing chain

My boyfriends car listed above 1.6tdci Diesel Zetec lost engine power whilst driving on the motorway (70 mph). The recovery mechanic said the timing chain was the issue. It was bought second hand at 145,000 miles and seems it had never been replaced. Now at 165,000 miles.

The garage has said they can replace the timing chain for £350 but it is a risk as it may have damaged the engine. The alternatives are to replace the engine or sell the car for parts.

Is there no way of telling beforehand whether the chain has damaged the engine?

Any advice on the best option is greatly appreciated.