I think everyone needs to accept some responsibility for there actions! The installer for whatever he did wrong AND your wife for the things she did wrong regardless of the time of day and where she was. I don't think the installer is responsible for the ruined engine, that was your wife's fault, again regardless of the time of day and where she was! She was the driver and is responsible for the vehicle. Ignoring the signs that there is a problem is not the installers fault.
Its amazing how many cars we (and by we I mean me) work on for the first time with hundreds of thousands of miles and after the work is done and the customer has left we get a phone call that my car has never done such and such and it must be your fault!
That's right those people have owned that car for years, driven it for countless hours and miles and we have it for 20 minutes and now everything is our fault.
The truth is when I screw up the customer is the first to know. I have no problem admitting my mistakes and taking responsibility for those mistakes. We all screw up, its a fact of life. Accepting responsibility for the screw ups is the difference between me and you (and by you I mean your wife).