I'd charge them, but with a twist: explain that, as it was obviously user error, and the company that you represent (and you do, even after hours) would be happy to bill them (more, likely). Plus, you'll be happy to credit (roll in) the charge on future work.

We just had to McGuyver a floppy center-speaker bracket on a customer-supplied speaker system. Normal hourly rate applied, and not a squeak of a complaint.