Whether or not to hire an attorney is a personal decision. I always recommend that someone at least consult with a local attorney about their problem since it is usually free and can leave you with valuable information and direction.
Does he have a lawyer? Do you know this other mechanic? If so, do you feel comfortable contacting him to see what was wrong with the vehicle? Do you feel comfortable calling the plaintiff and asking him for details on what was wrong with the vehicle? Do not admit to anything, but you need more information to know what your direction should be.
You need to gather information as to the nature of the problem. Then you need to ascertain if the problem was related to your work, to the parts used, or to some other factor that had nothing to do with the job. Without knowing more it is hard to tell.
At the very least you could send a letter to the plaintiff, or his lawyer, requesting a written description of the problems and the costs of each issue. This could give you an idea. Further, most new parts come with a warranty that may cover resulting damage if it was a part failure.
Please reply if I can help further.