legal concepts
Whilst being neither an expert in law French or English, there does seem to be two legal principles which do apply to the mechanic who applied the titanium strip
1. A frolic of his own. This archaic language means largely that the mechanic decided to do something outside of the known limits of what his actions should be. No company/authority can be blamed for someone doing something against instruction, unless the company knew and agreed or permitted the act.
2. If not for.... This is very plain. If not for the titanium strip being on the ground would Concorde have flown normally? It doesnt seem so.