On a scale from 1-10, how bad would it be list a full time position as an internship

Asking for a friend,

He had a job at a large corporation after graduation making 50k from June to August. He got fired/terminated in August for a pretty bad, but honest mistake. How bad would it be to list his experience as a internship rather than a full-time job on his resume, and on the background check to list it as a regular job. Anybody have any insight/advice on this?

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