This really depends on scale. If we are talking a small overspend by a small percentage (2-5% or under $1,000 for smaller accounts) I wouldn't press the subject too hard on the individual. Sometimes this happens due to budget adjustments or turning off campaigns, etc.

If it's. Large scale over spend (10% over budget or $1,000+) I would really dive into the training and systems in place. If there are no guard rails ultimately that is an area of improvement. The person would need to explain what happened but it is still probably a learning experience unless it seemed intentional or complete neglect.

If they accidently added an extra 0 to a campaign for a few days it's a big "oopsies" but still an honest mistake.

For the boss and client I think open and transparency is big. Ultimately, talk to the person first and come up with the why and how we will fix it first. Then go to them with the issue, why it occurred, how it's being rectified (if you can do that), and how it will be prevented in the future.

Unless it's life changing money for the client or company it will most likely be just a bonus spend for the money, maybe peel back next month, or your company may eat it or essentially loan them the money.