What you have to do with a Customer who a complaint ,if already one of your Colleagues was handling this complaint and fail to deal with him?

You Should Start Focus with the Customer on the main Problem , Check with him his Comment about the way which your Colleague have been used to solve his problem , and start make him Convince with your Products , or your Company Policy as your Trainer told you during the training , if you did effort you'll get profit

Working in team means supporting each other. To take initiative has never been a bad quality. Customer still waits for a response so it is in your hands to be solve it. In that case everyone wins - a team you work in, a customer, your colleague who has learnt on his mistakes, and you personally who has saved a face for a company.

nothing