There are a couple of interesting things in what the former employee says
1) He claims the commit logs are proof of when he did the work. They aren't, they are simply proof of when he did the commits.
2) He says, "I worked on it in the evenings after I had completed my work for the day." Which instantly makes me wonder whether he was doing this on a work computer after hours?
If you want to do open-source work on an area related to your job you must talk to your employer first, and whatever open source work you do you should be sure you're either not using company resources or have permission to use them. Commit logs cannot prove when you did the work, they can only prove you did them at or before that point.
1) He claims the commit logs are proof of when he did the work. They aren't, they are simply proof of when he did the commits.
2) He says, "I worked on it in the evenings after I had completed my work for the day." Which instantly makes me wonder whether he was doing this on a work computer after hours?
If you want to do open-source work on an area related to your job you must talk to your employer first, and whatever open source work you do you should be sure you're either not using company resources or have permission to use them. Commit logs cannot prove when you did the work, they can only prove you did them at or before that point.