Range Rover Tensioner?

I had the tensionser on my 1992 Range Rover replaced, along with changing the oil. I pulled the car off of the lot of the mechanic's shop who replaced it and drove five miles, at which point the engine overheated and blew a head gasket. Approximately $2,500 additional repair job.

Is there any possible relationship between the work done and the engine blowing a head gasket?