He's obviously not to blame for the choice of backboxes. However, whether you call it 'blame' or not, I still think that it's very unprofessional to do a bad job, even if that's due to third-party actions, without at least making some comment.Really she means "I just provided the backboxes that I had specified and supplied" He'd probably set aside a morning to second fix and made the best of a bad job. No excuse, but I really don't see that he is to blame.
Would you 'silently do the best you could' with what a customer had provided, and then walk away leaving accessories which looked a mess, without saying anything? Apart from anything else, to do so stands a chance of damaging one's reputation.
Kind Regards, John.