What kind of bike?

Frankly, it sounds to me like you didn't get the facts straight from the manufacturer. I'm having a difficult time imagining how a bottom bracket shell could have been installed backwards in the first place and then how somebody could have installed the bottom bracket backwards in it's place. I'm thinking that you have a different problem.

Italian threaded bottom brackets have a history of loosening during use. Some mechanics install them with blue locktite, some swear that torqueing them to 35 ft/lbs will do the job. All of the bikes that I normally work on are English threaded so I don't have the basis for a reasoned opinion.

Somebody mentioned green locktite. Green is for use after the threaded part has already been tightened. I recently used some on my son's downhill bike because the spokes were loosening up, but I haven't had the opportunity to check it since to see if the wheel will be true-able. He rides some big drop offs and does some things on that bike that I'm not sure that I approve of so it's a real good test bike.