First of all it is beyond the statue of limitations. If a Customer never approved for service then you are not held responsible especially if the client is outside of the warranty period or repair period.
A customer could come to you for an estimate or inquired about service but decide to go somewhere else as he/she did not approve(for you to perform the repair) for the repair. How can the customer prove that he or she did not damage it? How can you prove that you did not damage it? Basically it can be hearsay where he said she said. 6 years is quite a long time to come back for a free or discounted repair.
Lets say I brought in my laptop for repair at 123 Laptop Repairs 6 years ago and today I found out my hard drive is failing. I simply cannot prove that it was due to 123 Laptop Repairs as you are given a 90 day repair warranty and even then you could be charged for labor and parts if it was found out that the customer altered it. In the case of your customer they could have went on a Safari to Africa two weeks ago and broke the lens and wanted to find someone to blame so they decided to blame you. They may be trying to scam you because 6 years after the fact is too much.
If they wanted the lens fixed then they need to enter in a new contract with you.
Clearly you are under no obligation to enter into an old agreement as the cost of labor has gone up and the parts can be more expensive.
Remember the sign"We reserve the right to refuse service". If the customer is really pushing you to do something you don't want to do then you can tresspass the customer and tell him/her that they are no longer welcome in your establishment and if they come to you again you will call the police and charge them for tresspassing.
Having a tresspassing warning or order could be helpful to you if you do go to court provided that you are doing it legally and not discriminating against the customer.