That wasn't a root cause, it was a restatement of the problem. Also, as you hinted, there is no systemic change to ensure it won't happen again. I'd ask for them to go back and try again. If training their people didn't prevent it the first time, ask them why retraining (the same action) would prevent it now...
As far as a system audit, here are a few things to consider:
What would be the objective of the audit? In other words, what are you hoping to accomplish with the audit? Have they been a good supplier in the past except for this incident?
Also, what requirements would you audit? Do you or someone else in your organization have the competence to audit ISO 17025? Or would you audit them to ISO 9001 and their own documentation?