Hi Pete,
If there are no portal users in a customer account, they don’t need a password at all. But if they do have a portal user, and passwords are required, then no, there is no way to shorten the minimum length requirement. This is intentional in accordance with best security practices. You might suggest they use a password manager like LastPass or 1Password to help with this. Many browsers can also remember passwords for you as well.
I just revised the Customer Portal article that covers these things in more detail than before. Please check out the first two sections and let us know if you have additional questions.