This appears to be an issue with proper training and a failure in supervision.
The welder should understand the differences between E7014 and E7018. That is a training issue.
The issue with supervision is another issue. We don't know all the details that would point the finger to someone that didn't do their job.
Was the welder a contractor working on a job site? If he was, did he have his own or was he provided with a WPS that provided the needed information? Was this job governed by a code such as an AWS structural welding code or ASME construction code? If the project was required to meet a code, who was responsible to review the WPS the welder used (or didn't use). Was the welder an employee? If the welder was an employee, who was the immediate supervisor?
The issue I'm attempted to circle around to is what is going to be done to ensure the same problem doesn't happen in the future. Additionally, what is going to happen to rectify the mistake that did occur. If the project is required to comply with a code, there may be ramifications other than whether the electrodes are equivalent. The simple answer is they are not equivalent in all respects. Many of the differences have been noted, but one has not been addressed. That difference is the F numbers assigned by either AWS or ASME. The two electrodes have different F numbers. The F number is an essential variable. Ouch! Again, if the project is required to comply with a code, there are code issues that have to be rectified.
It would be nice to know the whole story rather than interesting tidbits.
Best regards - Al